I think we're talking past each other a little. I'm agreeing with you, I think.
Kellhus had a much broader range of breeding stock, imperial concubines are mentioned. Like an empire worth of choices. He has people who will conduct any secret mission and happily top themselves after doing whatever task he asks of them without question.
We agree here, I think?
Maybe Esme was the only other one who could produce viable children (albiet with less success), but it wasn't for lack of opportunities.
We agree here, I think?
Um, how many women did blind Moenghus/Mallahet the Priest have to choose from?
...
Seriously ... he managed to find the only other worldborn woman able to bear dunyain children?
I'm on a sabbatical from work at the moment and I took a little vacation: a friend of mine who hovers here and I had a conversation, which featured this exactly (aside I recorded it as I'm thinking about breaking into unrelated podcasts but I wanted to test my editing skills).
Moenghus the Elder arguably found at least one woman, if not more than one, with which to breed (arguably) viable offspring (also, we don't know what proscriptions Mallahet "broke" as Cishaurim to do so).
But yet Kellhus who can exploit all the advantages in selection, which you highlight, can only find
one other woman to produce a child that is physiologically normal...
So Moenghus and Kellhus, arguably, found the only
two women with which to breed with...
I think we're both wrestling with the same incredulousness here.