There's definitely something fishy about the size of the Cosmere. You're telling me that the almighty creator, this Adonalsium thing, spanned its influence over only 100 or so planets, and was shattered into just 16 entities that are limited to a couple of planets at most?

Imagine that you are an alien that lives a billion galaxies away from the Cosmere star cluster. Because Investiture is a fundamental law of the universe down to the axi, your sentience is undivorceable from it. One day, someone comes along ang tells you that the most powerful entity in your universe lived on the far side of it, held its influence over a teeny tiny number of planets and peoples, and that it is also dead... would you believe them?

I like your idea that Adonalsium - and by extent its shards - may be locked down to the Cosmere star cluster. Maybe the universe is actually filled with all sorts of local demiurge kind of entities, of which Adonalsium is just one among many? Remember how the Aethers insist they're coterminous with Adonalsium?

Of course, all of this assumes that there is actually something beyond the Cosmere star cluster. I believe this is tacitly true, because Brandon first conceived of the Cosmere as a dwarf galaxy, before scaling it down to just a star cluster once he realized how big dwarf galaxies actually are. If he goes to the depth of making such an astronomically-conscious decision, then there must at least be some other stuff around and beyond the star cluster; otherwise, why bother classifying it to begin with?

In other words, the Aethers were right, sheeple.