Dunkum

I doubt that would quite work. it may be possible for the collective imaginations of the various sapient species on Roshar to create a Nightwatcher or Stormfather level being - maybe even something on the level of a shard, if they can all sort of focus on one thing long enough to pull most of Honor's investiture back together - but there is just not enough investiture in the system for them to create something at Adonalsium's level.

1. probably: Hemalurgy is pretty versatile, an Atium spike in particular, so transferring investiture or connection or both should be doable.
2. hard to say. investiture resists investiture so it may nt be possible for a spren to bond to a Shard. moreover it isn't like there would be any benefit to the shard for doing that, they are almost infinitely more powerful than a spren alrady
3. no reason they wouldnt be able to in theory, as long as they were compatible with the shard in question, but as to what happens to the spren, it is hard to guess. depending on the mechanics of the bond it could go any number of ways, some of which could be benign, others of which could be potentially harmful.

A couple of things:
1. storing/tapping is additive, not multiplicative.  this is easiest to demonstrate with Iron/Mass since the units make more sense: if you have a mass of 60 Kg and you store 1/2 your mass for 1 hour, then for that hour you have an effective mass of 30 Kg.  if you then tapped that you would not have double mass for an hour, you would have 1.5x mass for an hour (regular mass + the 1/2 mass you stored) so 90 Kg.
2. moving from that, then, the most you can store of an attribute would be 100%; and storing 100% for a second would let you tap 100% and double whichever attribute for 1 second. there is no such thing as storing an infinite or near infinite amount as you approach 100%.
3. however even that theoretical limit of 100% isn't truly achievable for most attributes. in a lot of cases storing 100% of an attribute for any length of time would result in death.
4. and finally - there are diminishing returns on over-tapping (Brandon originally has Sazed call it compounding in era 1, but that term has since come to mean something else, so i'm going to call the act of tapping a lot of an attribute in a short burst over-tapping since I don't see an existing term for it).  to go back to that mass example if you stored 30 Kg for 1 hr you could tap 30 Kg for 1 hour or you could tap 60 Kg for maybe 25 minutes - strictly less than the 1/2 hour you would expect.  so while there is an actual multiplicative aspect here, the more you try to tap at once, the more of it gets wasted on the extra expenditure to do that over-tapping and the shorter the burs of extra attribute gets.

I think you are basically correct: a coppercloud won't prevent a tineye from hearing what is going on within, because a coppercloud blocks investiture not physical sounds.
to your other question, tineyes still have limitations, and soft speech blocked by walls, or surrounded by other sounds will be harder and harder to hear.  A good tineye might be able to pick out one soft conversation amongst dozens, an a savant could probably manage it, but just your average tineye would have trouble.  likewise for walls and additional distance - the more obstructions and distance between the source of the sound and the listener, the harder it is to hear, same as for anyone, really, its just that the tineye has a better baseline because of their enhanced senses.  Most of the crew have experience with mistbron and tineyes, and would generally be familiar with what a tineye's limitations are, and would probably be able to keep themselves unheard by any but a savant.

putting this in spoiler tags just in case, though I don't think it probably needs it.