Not sure on Yatwer genetics, seem more technical than fantasy.
Typical defective Dunyain is an interesting idea. I doubt the Dunyain would have been able to survive 2000 years if they had the same frequency of birth defects as Esmi has had. Though it is possible that it has slowly gotten worse over time.
Maybe Moenghus was sent out to see if the worldborn women had the same problem bearing Dunyain children.
If it where my money, I'd still put it on genetic drift until we get more info. The particular set of dominant/recessibe traits, as well as strickt breeding controls, might have been enough for the Dunyain to suppress breeding issues. Who knows, maybe without those careful controls the Dunyain would have similar defects as with Esmi's children. Kellhus sees it as worldborn women being inadequate, but maybe its just the screwed up Dunyain lineage.