Since synthetic gems would work for holding stormlight, then gems from other planets should work too--taking into account that they might not be as "pure" as Rosharan or synthetic gems. Like how pewter is an alloy that can have varying percentages, but only a specific one will work, and the farther you deviate from that, the less power/more problems you'll get.
Can you cite the source of gems being able to refill from any perpendicularity? It makes sense but I can't find it at the moment.
I'm pretty sure you can't soulcast *to* another polestone. If that were possible, Diamonds would be worth almost as much as Emeralds, since they would then be able to use Diamonds to make Emeralds, instead of being one of the least valuable.
It appears so.
I think the Parshendi gemhearts are something different that we don't know much about yet. They don't seem to actually store stormlight so much as store spren. I mean, if they did store stormlight, the humans would be harvesting their bodies for the gemhearts, right?
Unknown on the gemhearts/coal, etc, thing. I think they'd be a bit easier, but we won't know until/unless that happens or someone can get Brandon to tell us