Like you've said, there's Fifth Magic woven into this - selecting people from different parts of a timeline - and obviously, the Second, since moving to another world truly is part of this. The reason I mentioned the Third earlier was because of how we've been brought in - summoned, I'd guess, although strangely and somewhat incompletely. Alternately, we're being controlled.

[come, Waver. sit with him, because he's going to take up space under a tree and let one of the nymphs crawl on his arm.]

There's a system I know of that could take a number of souls to another place, another part of the timeline. But shifting that many to a new world, as appears to be the plans...that's different. So many interlocking points.