Pagerunner

There were two different spren in the flashback in question. First was Sja-Anat's spy, which was described as a face with red eyes or as a small red dot while it was moving. And second was its "escort," a shadowy humanoid figure, six feet tall, which is the one that dove into the ground and became a thunderclast.

It's the beginning of The Final Empire (emphasis added):
 

Here are all the WoBs on the subject (we actually have a tag for this character!): https://wob.coppermind.net/adv_search/?tags=kandra+worldhopper

Nothing further. According to the original report, it's not the 'survival' Shard. We also know that it was not referring to Ambition. So, we're still right where we started, no further hints.

The question was indeed asked of Brandon on the Arcanum Unbounded tour, and it is a coincidence:
 

An excellent question, and one that I've been considering since BoM was released. I don't think we have official confirmation either way. Right now, I lean towards a copper-like interpretation, both because it was my natural assumption when I first read the story, and because of how I currently understand the relationship between Investiture and a spiritweb that produces magical effects in a user. I don't think that the Investiture stored in the nicrosilmind is a consumable, the same way that a store of weight or speed would be. They need to refill the brass portion of the medallions after use, but maybe they don't need to refill the nicrosil portion.
That being said, there are also good arguments that say it's a time-dependent aspect, that you store 50% of your Allomantic strength for an hour to let someone else be a full-strength Allomancer for half an hour. What we've seen in the canon isn't detailed enough to specify either way, and Brandon has been quick to hand out RAFOs on the exact function of the medallions, so as not to spoil any reveals coming in the final Wax and Wayne book.

1) They were actual atium mistings. Because it was a signal (16%), Preservation was only keying in those metals that people knew as he snapped people.
2) We don't know. I, personally, think it's likely; he already knew Kelsier personally, so I don't think being able to interact with him later would cause his faith to waver.
3) The Ten Essences are a Rosharan philosophical concept. I guess it's possible that they are derived somehow from the interactions of your physical body with your spiritweb, or sDNA; I don't think anyone's ever thought about that before. It wouldn't be a direct connection, but they might build from the same Realmatics. Excellent question.
4) Doubtful. The magics of Sel are tied to the physical regions where they occur. If they could somehow be tied to the planet as a whole to produce the effect you describe, then why would they be restricted to regions in the first place?
5) I think Hoid can take care of himself, with or without emotional allomancy. He'd probably get a kick out of someone killing him, losing their status, and then he'd just show up and become his own replacement, claiming to be his own twin brother or some such nonsense. He did use some Allomancy in one of the WoR flashbacks, however, so it's definitely a possibility.

Have you checked out the Theoryland interview database? It's primarily a Wheel of Time site, but they do still update with Brandon's signings, so they've got a lot of juicy tidbits that were obtained at signings that you can't find anywhere else.
If you look in peoples' sigs, you'll see some of us have an "Espoused Theories" section. If they're following the suggested format, anything without an asterisk would be someone else's theory that they like. If you see a theory popping up in multiple peoples' sigs, you know it's a good one.
You could use the search bar to filter by forum (Cosmere Theories has more overarching ideas, while for now Edgedancer/Secret History spoiler forums tend to be the most cutting-edge about those topics) and search for a topic you're interested in.
But, probably, just start clicking away through a forum. If you find someone referencing a theory that you're not familiar with, quote them and ask for more info. I doubt anyone will complain about being 'forced' to talk some more about an idea of theirs. Posting theories of your own usually brings us out of the woodwork, saying "Cool idea, I think that fits with my theory, too."

Welcome to the Shard! The earthquake in Arelon that caused the Reod is a fairly recent event in the cosmere timeline, thousands of years after Odium splintered Dominion and Devotion. So, it wasn't caused directly by him, but it could have been an aftereffect. The splintering of Honor, however, seems to be a much more recent occurrence, since Dalinar's visions show the Radiants giving up their powers, an event remembered by the people of Roshar while the original deaths of Aona and Skai were in Selish prehistory.
Of particular note is the pattern to the Shattered Plains. Some of us find it reminscient of the cymatics that Kabsal showed Shallan about how the patterns mapped out several cities. That's probably the stronger connection - whatever vibrations caused the patterns that form the cities, could have been amplified at Stormseat to create the Shattered Plains.
So, yes, I think you make a good point when it comes to Stormseat possibly being destroyed because of a lot of Investiture. I don't think we can tie it back to Elantris, however, because we don't know exactly what caused the Chasm to form. (Although, it is interesting to note that the Fjordell prophecies say Jaddeth will rise from below the ground.) But it's a good idea, and a good parallel to note. I definitely agree that both are likely due to Odium's influence, if not through the exact same mechanism.

That's actually a really interesting observation. It's not the only time that you'll see something a little anachronistic; I think Wax and Wayne have a lot of metaphors that don't quite make sense given the history of their world. I expect it's for the benefit of the readers (especially the casual readers), so not throw off the pacing of the story. Brandon plays with that concept in Stormlight, with Wit intentionally making references to creatures that nobody on Roshar would be familiar with and totally derailing the conversation. But in terms of the text of the book, it can be treated as a 'translation' to English, with phrases like that meaning more like "the planet's final sighs" said in a way that seems natural to the readers. It does use earth with a lower-case-"e," so it's not specifically referring to our home planet.