Sasha's explanation sounds logical enough to me, so I'm fine with that. I'm thinking that Amalor was pretty much a colony used by lots of different races all the way back to prehistorical eras, and may have just been one of the provinces in the Dark Elven empire of the time (maybe not even seen as all that important, so they didn't show up in the other description). Makes sense that the Dark Elven survivors would have left, given the violent disaster of the colony.
When I described the downfall of the Dark Elves of Amalor, I only meant the ones in the colony, not the entire world, so the survivors may, indeed, have departed and only a few might have returned much much later.
As for Dark Elves and Shadow Elves, I was wondering the same thing. Is there a difference? And if so, what is it?
In any case, the Dark Elves of Amalor may look different than the Dark Elves of the rest of the world (for one thing, mine are going to have black hair, not white).