Weltall

Yeah, limbs rendered inert by shardblade wounds don't rot and the whole thing works because of Cosmere metaphysics.
 

A polestone drained of color by an Awakener would become useless for soulcasting or for use in fabrials but it would probably still be able to hold stormlight albeit not as well. And this would work on diamonds too.
 

Yeah, ingesting a feruchemst to mimic their appearance would do nothing to grant a kandra feruchemical abilities. Now, if you had a hemalurgic spike, a kandra could use that (it's very easy for a kandra to use spikes) and then use any metalmind the original feruchemist filled with the corresponding trait or fill their own metalminds, because the spike steals enough Identity to make them the same person, as far as a metalmind is concerned. But that's a separate application of magic.

For a multi-Shard magic system to emerge you need multiple Shards on the same world. We have examples of this on Scadrial where Preservation and Ruin each have their own systems, with feruchemy being the shared one. Roshar has three magic systems (or thirty, in the same way that allomancy could be thought of as one magic or a whole bunch per metal) and all of the magic systems on Sel are mixed Devotion/Dominion with some leaning more one way than the other.
On Roshar, Surgebinding is primarily associated with Honor even though Cultivation is involved with it. Look at how you get access to the magic and you'll see how Honor-centric it is, and we know the Radiants of old could speak to Honor but we have no evidence they were as closely connected to Cultivation. We know Voidbinding is the second of the three major magic systems on Roshar and that's of Odium, which leaves a third system for Cultivation. Khriss suspects in the Ars Arcanum that the Old Magic isn't actually a system and Cultivation's magic is something else. Since the only use we've seen of the Old Magic is the Nigthwatcher (a godspren) and Cultivation providing similar effects, it's quite likely that the Old Magic is indeed something unique and not the third system.
All other magic systems we've seen in the Cosmere are the result of a single Shard, or are magics that could arise without the direct intervention of a Shard even though they're associated with one by default. Awakening on Nalthis is an example of the former, the Aviar on First of the Sun of the latter. Shards can also be involved with multiple magic systems across worlds; Sand Mastery on Taldain is a directly accessible form of Investiture but there's some other manifestation on Darkside that we don't know much about, and given the number of worlds she's involved with it's pretty safe to assume that she's tinkered with magic on plenty of other worlds even if she didn't have a hand in creating the systems. Or to be more precise, shaping them. Brandon's mentioned several times that Shards don't truly create the magic systems they power, they just influence what arises organically from the interaction of their power, the world(s) and the sDNA of the inhabitants of that world. Also, that there are natural pathways that the magic follows which is why Lightweaving exists on both Yolen and Roshar.

I think that anyone who already has the (s)DNA for Radiant eye colors through heredity won't undergo a change if they become a Radiant themselves (Brandon's 'they were already light' comment) and they'd pass on whatever eye color they were born with to their children. The caveat to this is that we know that eye color genetics are really funky on Roshar and a child can inherit either parent's eye color or get one of each.
So based on that, if a 'natural' lighteyes and someone who became a lighteyes through the Nahel Bond had children, there's a chance that those kids could have either Parent A's 'natural' eyes or Parent B's 'Radiant' eyes. The former would be permanent, the latter might not be. Brandon mentions that people like Shallan are the product of a long line of people who had the changes and eventually the alteration stuck, so someone who inherited 'Radiant eyes' from a parent rather than 'natural' light eyes would probably still have sDNA that's malleable enough to undergo a color change if they themselves became a Radiant.
Likewise let's say Kaladin and a darkeyes have a child. That kid might inherit Kaladin's Radiant eyes instead of their mother's dark eyes but would probably still be able to change to something else if they became Radiant themselves.
For someone who's already a lighteyes and becomes a Radiant, since they already have the sDNA for one eye color, they'd pass on whatever color they were born with, in the same way that their eyes don't change. Whatever part of the spiritweb controls eye color on Roshar has already been altered so the changes that the Nahel Bond would normally impose wouldn't have anywhere to stick, and thus wouldn't be passed on to the next generation.

Yes. It's theoretically possible for something really funky to exist if Adonalsium or a Shard wanted it to but in general worlds in the Cosmere form the same as they do in our universe. The most extreme it gets is Roshar (whose moons aren't stable in the long term) and Taldain (also unstable across astronomical timespans) which bear the touch of Adonalsium but are stable enough on the timeframe that Brandon is concerned with.

Welcome to the Shard!
As mentioned, Brandon is very clear that he's never going to provide a canonical explanation for what the Beyond is (or even that it exists) and at most we're going to get what in-universe characters believe about it.
Which means that nothing that happens in the books is ever going to be so directly involved with the Beyond as to give the reader a definitive answer. The comment at the end of Bands of Mourning about serving Trell in another realm (which I assume inspired the question) could be a reference to Cognitive Shadows which don't directly involve the question of the Beyond, or it could be something more along the lines of what Moelach does in Stormlight Archive where the transition between Realms serves as an opportunity to gain some insight into the future.

As a general point for you to bear in mind in the future, if Earth appears in one of Brandon's books, it is not set in the Cosmere. Brandon has said that it causes too many problems for any version of Earth to be included, even a really funky alternate Earth.
And welcome to the Shard!

You'd be trying to affect Investiture with Investiture and those interfere with each other, so no. There are situations where you can overcome this interference but for all practical purposes what you're asking isn't going to happen.

1) The intents of the Shards aren't related to what you can do with the magic but how you access it in the first place. Allomancy lets you preserve your own strength by drawing on Preservation's power, hemalurgy results from an intent to ruin someone else, feruchemy lets you ruin yourself in the short term and preserve your strength for when you need it.
2) The intents of the Shards are not inwardly-directed, which is how Preservation can 'give' Investiture in the first place. Ruin has no problems giving a tiny bit of his Investiture to power hemalurgy (which results in a net increase in entropy) or however it works in feruchemy, since that evens out to neutral but doesn't really go against his intent either way. End result is still going to be the same.
3) Ruin isn't 'just' entropy or destruction, he's capable of creating for the purpose of destroying. Sazed puts it well in one of the epigraphs.
 

It's technically possible but would be difficult in practice. Presumably this has to do with a combination of potentially conflicting Ideals and the way that a Nahel Bond fills cracks in the Radiant's spiritweb possibly making it harder for a second Bond to be formed.
As far as Squires go, we know that it's possible for someone to be a Squire of one Order but eventually become a Radiant of a different Order. Whether you could become a Squire to someone from a different Order after already attracting a spren of your own hasn't been addressed but I imagine it's possible in theory, the same way that bonding two spren associated with different orders is possible in theory.
 

While it's not canonized yet, Brandon's current plan for how humanity got from Ashyn to Roshar involved the Spiritual Realm, which.rather suggests that it is a feasible form of transportation... if you know what you're doing. There's also Aon Tia and the Dakhor monk's methods of teleportation which are point-to-point and so probably send the users through the Spiritual. We know the former requires you to be extremely precise if you don't want to end up in the middle of the ocean, or a rock face or something equally unpleasant and we don't know enough about the latter vis a vis how you specify a destination but since all Selish magic obeys the same principles it's probably similar. So yeah, Spiritual travel for non-Shards is feasible but it seems to be considerably more complicated than hopping over to the Cognitive and walking, and potentially more dangerous.
@CrazyRioter hit on the precision risk but there's another possible factor at work and that's how the Spiritual appears dangerous to look into with the unprotected mind. Leras tells Kelsier that spending too much time looking into it could break his mind and the sensation was clearly unpleasant. Transitioning bodily into the Spiritual as a method of travel is probably even more dangerous even before factoring in the extremely high probability that any screwup in travel will leave you in a vacuum.

Sixteen is important to the wider Cosmere because it's the number of Shards of Adonalsium. There are other significant numbers but these are strongly implied to be associated with planets rather than the Shards directly.

Well, the exact appearance would depend on the Cognitive region but using Roshar as an example because we've seen it the most, the flames that represent living things would all vanish and the beads representing inanimate things would change to reflect their new Physical state, though anything outright disintegrated would probably vanish quickly, in the same way that the Wind's Pleasure would have been subsumed into the ocean rather than remaining 'a bunch of water that used to be a ship' Cognitively.
As for the terrain, I imagine that would take longer to show any change. People on the Physical side would have to begin thinking of the area as being different before it would alter the corresponding Cognitive area.

This is easy: Either Khriss or Hoid because then I'd Know Things, the likes of which I'd give an arm or a leg to know right now. xD
Setting named characters aside, being a member of the in-universe Seventeenth Shard or a scholar in Silverlight would be awesome.
When you're a Vessel, you really don't get a choice. There's some wiggle-room to interpret the intent of a Shard but you can't get away from the essence of it. And if you're not well-matched with the Shard, you'll get twisted by the intent far worse than if you were. See what happened to Ati compared to how well Rayse is coping with being the embodiment of divine hatred.

Shinovar still gets highstorms (and thus stormlight) even though they're not nearly as violent as in the lands further east. We know they've got plenty of access to stormlight since Szeth trained with the Honorblades long before he was declared Truthless and that requires a good supply of infused gems. Szeth does think that the Alethi custom of using infused gems for mere illumination is profane but that doesn't necessarily mean they don't do that in Shinovar. Until we get a better sense of Shin beliefs we can't really predict what would be considered an acceptable use, but I can imagine a couple ways that they could use stormlight lamps without falling afoul of whatever strictures Szeth had in mind when he made that thought.
Plus as mentioned, fires are easier to start on Roshar but they're not massively dangerous and we've seen plenty of examples of fire being used throughout the books. Humans have known how to control fire without burning their homes down around their ears for millenia, there's no reason for it to be any different in Shinovar.

We know that storing Identity opens you up to all sorts of things in the Cosmere and it would be easier to affect you with a soulstamp, so I imagine that someone who's currently storing Identity and tries to burn gold would get some very weird results. On the other hand, tapping Identity while burning gold would probably do some cool things. Maybe it would make the process less unpleasant, or the 'extra Identity' might extend how far back into the past you can look with A-Gold or give you more finesse over what you see.
They could, but that's with F-Gold and A-Aluminum which is the opposite of the hypothetical twinborn... though I imagine that if medallion tech spreads there would be a huge market for ones granting those powers on Threnody. Actually there would probably be a huge market for F-Gold medallions in general, for obvious reasons.

Welcome to the Shard!
Hoid is not a Returned though he has at least enough Breath to have reached the Second Heightening. His healing is something else entirely:
As mentioned offhandedly in that WoB (and implied in his appearance in Mistborn Secret History) Hoid is physically incapable of harming people to the point that even contemplating inflicting physical harm will nauseate him to the point of incapacity. Additionally Brandon has said that Hoid's spiritweb is weird, moreso even than a savant. He was human once but now he's not, exactly. He's also really truly unfathomably old. Like, older than the Shards old.
In short, Hoid is... Hoid.

What he probably did was force the Stormfather to manifest Physically just enough to serve as an Oathgate key and the latter was so surprised that Dalinar was doing it that he wasn't able to stop it from happening. We know that Bondsmiths did not have Shardblades historically so it's unlikely we'll ever see Dalinar with a full blade and we'll probably never see a repeat of that scene either.

Yeah, there's no biological equivalent on Earth but since we already know of aluminum which acts like terken and we also know that it goes by other names on some worlds (it's the metal called ralkalest on Sel) the most parsimonious explanation is that there's aluminum in the carapace which gives it its anti-Sand Mastery properties, instead of being some new material that just happens to act in exactly the same way. There's also the obviously artificial nature of the Taldain System to consider, so the fact that aluminum shouldn't naturally be a part of a creature's carapace can be explained with 'Adonalsiumdidit'. Or possibly Autonomy.

Here's a list of all the books (including major planned but unwritten works) and their rough timeline, for some details.
As far as time travel goes, we've seen multiple characters make use of time dilation whereby they don't actually experience all of the years they've been alive in absolute terms and so live longer than they should be able to. We know from Word of Brandon that Marasi (or any Pulser) with a sufficient supply of cadmium could also push themselves a good way into the future with just their powers. However, it and all other methods of 'time travel' we've seen are a stricly one-way process moving foward, not backwards. Brandon has conflicting WoBs on this (which may be the result of paraphrasing more than any inconsistency on his part) but he's answered pretty definitively on occasion that travel into the past is not happening and we shouldn't expect to see any serious time travel shenanigans in the Cosmere. Other WoBs are more open ended and have said that nobody has figured out how to do it, but that one is pretty definitive and it's verbatim.
Now, time 'travel' via the Spiritual Realm is something else entirely and is definitely possible without violating anything Brandon has said. We see a form of it in Mistborn with A-Gold and A-Malatium showing you the past and in Warbreaker Lightsong had a vision of a key event from the Manywar centuries before. Someone able to peek into the Spiritual might be able to 'see' the past but they couldn't bodily travel there and affect things. We also see some screwy things with Syl and Kaladin's bond manifesting before the relevant Physical events happened. Kaladin's spear proficiency and Syl taking the form of Shallan at the beach (well before the latter first met the former two) are illustrative of how the Spiritual Realm can be brain-breaking.
For the worldhoppers, there's a method that most of the Seventeenth Shard uses (Demoux included) which greatly slows the aging process but doesn't stop it. Brandon has said that there are many methods, this just happens to be one of them. Other ways we know it can be done include attaining the Fifth Heightening (stops aging completely). compounded atium (which has an upper limit and is noted to be inefficient) and becoming Invested enough that you become a Cognitive Shadow upon death, then figure out a way to reconnect to the Physical Realm ('The Sovereign'). And then there's naturally immortal (or at least very long-lived) peoples like the Siah and Dysian Aimians, the kandra and dragons.

It wasn't the amount stored per volume of gold that surprised him, just how much health there was. Bloodmakers have it very rough because you have to spend a lot of time feeling awful and you can burn through stored health extremely quickly. After his big expenditure of health about halfway through Alloy of Law, Wayne mentions that he's storing as much health as he can while still being able to move around and by the end of the day he'll have enough to maybe heal a scratch. He normally needs to spend weeks in bed to store enough health for useful healing in an emergency. So what that bracelet represented to him was more stored health than he'd ever had access to, free for the taking.
Oh, and we have this WoB about metalminds that's relevant to discussiing Era 2.
And more generally, this one that talks about how volume of metal relates to how much of a charge it can store
 

Oathbringer Spoilers:
 

Brandon has said that Larkins and Nightblood are similar in the way they leech Investiture. He's also described Nightblood as being an order of magnitude more powerful than an ordinary (living) Shardblade. Now consider the following WoB:
Ergo, it may not be possible to drain the Investiture from Nightblood at all and if it were possible it would be really really hard. And Nightblood would be resisting so it's likely the end result would be the Larkin fleeing in terror, deciding that Nightblood is a god and beginning a cult dedicated to it or getting imploded by Nightblood trying to leech it.
Also, Brandon has said that Nightblood isn't powerful enough to kill a Shard, though he's been deliberately vague on just how much damage it could do to one. There's no way a Larkin could do better than that and it would probably get splatted like a bug if it tried. Assuming you could even get a Larkin into a position where it could try to leech a Shard, given that they're mostly in the Spiritual Realm.

My guess is no on both counts but for different reasons. The 'charge' in a spike is essentially the bit of spiritweb that it's ripped from the victim and there's a bit of Identity coded to it. In other words, you don't get 'the ability to use allomantic steel' from spiking Coinshot Joe, you get 'Coinshot Joe's ability to use allomantic steel'. This is why it's possible for someone with feruchemy gained through hemalurgy to use the metalminds of the spiked person; they gain enough of the Identity of the original feruchemist that the metalmind recognizes them as the right person. Spiking a second person with an existing hemalurgic spike would be adding a second independent charge rather than a suppliment to the first one, even if it was the same power. I'm not sure how the spiritweb would interpret that if they came from the same spike, as opposed to coming from separate spikes.
On the other question, I don't think it would work physically. In all examples we know of the spike needs to go completely through the victim in order to pick up whatever trait it's stealing, so even if you had a super-long spike in your body and managed to poke somebody in just the right place, I don't think you'd be able to trigger its spiritweb-ripping-off function.
In both of these cases, another issue to bear in mind is that since the spike already has a charge (and a distinct charge with associated Identity), they're already Invested objects and it would be harder to get a second charge into them. We know it's possible for objects to be so invested that they can't really be made into spikes on account of being 'full' already (even if the example's we've been given of this are super-Invested things like Nightblood) so a small spike might not be able to hold a second charge anyways.
Just found a WoB that seems to leave a little wiggle-room for multiple charges in a spike, but in general the answer appears to be 'no'