Well, there Kellhus would have to weigh viability against secrecy (which is improbably rare for worldborn women with full-Dunyain - opposite, we don't even know, for instance, that Serwa can bear viable offspring). I imagine he might have tried a couple times with the first few concubines - isn't it the case he turns to surrogates after Esmenet's nameless one?
I can see that being the case, yes, but as Aspect-Emperor Kellhus didn't exactly have a shortage of possible concubines that could potentially have the right genes to make it work. So they'd still be sort of expendable.
It can also be that the first births (resulting in deformed children) were very difficult and the complications rendered them infertile. Esmenet almost died giving birth to the "nameless one", for instance (granted, she was able to have three more children after that, but it could have gone the other way too). In a world without modern medicine, that could have been the case for at least some of the concubines who survived. (Now
that would be an interesting POV to have, that of a surviving former concubine of Kellhus)
I think Esmenet actively tried to get Kellhus to turn to the concubines after Inrilatas' birth because she didn't want to have any more children. He might have been trying to get more children by women other than Esmenet
before this point, though, it's not fully clear in the text.
You bring up an interesting point with Serwa. That is something that we'll probably never know (and that I'm extremely curious about), whether half-Dûnyain would or not be sterile. Even if she
could have children, it's not like she'd be having any after the No-God rose, after all.