I've wondered about whether 8 was the magic number needed for the rejoining. It seems implied, since that was all the elements, but never been stated that I recall. I believe it is, but at the same time the story given makes no mention one way or the other.

I assume the sacrifices made to bring them back would have been made similar to how we've seen Primal summons using crystals. Death isn't the factor here, releasing the aether is.

Too hard to say. Time travel does weird things to story ideas.