Well, all this
could be support for an idea I had during the Slog, which is, that what the Dunyain thought they were breeding and selecting for isn't actually what was
really being selected, it was really just confirmation bias in action. I am still of the mind that Dûnyain are really a hybrid race, a mix of Man and Nonman and the distillation of the Nonman blood is really what was going on, not the pushing of human limits.
In addition, there is also the fact that we have talked about before, how Kellhus' journey, especially at the begining seems to
improbable. Hell, he nearly dies to
snow. Perhaps the real selecting agent really is something from the Outside. That all the breeding, all the training, was all just because something needed Kellhus,
specifically.
Of course there is the chance that bad biology is just a thing in Eärwa. That for whatever reason, genetic rules are real there, perhaps for the same reason that Gods are real, or damnation is real.
My guess though, from the text is that the women, at least at first, were willing to submit to being "bred according to the fitness of their progeny." Additionally, "over the centuries, the sexes were transformed," he said, "each according to their share of our burden."
To me, putting these pieces together, I get the feeling that the Dûnyain had no real idea what they were doing with genetics. They were simply guessing what combinations could possibly be best. In other words, since they couldn't make sense of the permutations (or at least, they probably thought they couldn't, because it seemed too random) that come with sexual reproduction. Either that, or they really didn't want to be bothered, instead they would rather have removed the "trouble" of sexual politics all together.
It kind of goes to something that Kellhus says:
“Men,” Kellhus said, “cannot dominate their hunger, so they dominate, domesticate, the objects of their hunger. Be it cattle …”
“Or women,” she said breathlessly.
So, this was probably where Kellhus gets the idea from, the Dûnyain had done it. They had dominated and domesticated the women, to the point of them basically not being human any more.
Indeed, this explains why there were still defectives (and seemingly commonly) and why they bred so young. Because they were just forcing generation after generation, constantly, until something (someone) came out
right. Until some specimen was essentially all their father and almost none of their mother.
Plausible?