A lot of the questions you have are addressed in other books that are set before this in the timeline, so if you'd read them you'd have a better basis. Not saying that HAVE to have read them - I assume you still got the general gist of what was going on - but the fertility explanations and world-building you're looking for are in those other books, particularly the ones Dimesdan
mentioned. I would guess that they weren't addressed quite so heavily in PoD
just because they'd already been established earlier and the writer didn't want to repeat things.