Alteima comes back in ToM, actually, but you have to keep your eyes open.

Maybe worldbuilding isn't the right word, exactly, but "epic sweep" may be closer. I think at some point Jordan decided that since he now had the luxury, we were going to see this snapshot of time in Randland from the eyes of more than just six characters. It's sort of like how they used to have those epic books about one place that were a thousand pages long and covered a million years (wasn't Robert Mitchener one guy who did those?) except epic fantasy and even longer. I don't know; we'll never really know, unless Harriet starts talking, which I doubt.

I don't think the depopulation has a very satisfying "explanation." I think it's all just an authorly handwave signifying a "declining age" slouching towards Tarmon Gai'don. As for why Ishmael would dastardly do it, well, a weak, fractured, underpopulated Randland is really good for the Shadow's chances of winning by sheer weight of Trollocs. Also, there seems to be some sort of metaphysical component, too, where "unity gives strength" and the Shadow's power wanes where there is unity and order.