Yeah, I hardly think Kellhus has made such a straightforward and serious mistake as screwing the Niom.
Socially, functionally, Moenghus is his son. Perhaps he wants the Niom to fail though, ldk.
The real question about the Niom is why they didn't return via Quya Chariot? Why do they have to sneak up on Ishteribinth? They have made ridiculous speed there by teleporting, so how will they explain that, especially if they beat the chariot there. Plus the ordeal won't have taken Dagliash by the time they get there (and word of that would have to travel too, so why send the kids now if that was the stall on getting a lift?)
Why did Kell select Sorweel as the 'enemy' - if Kell was fooled by Yatwer's mask then he shouldn't have picked someone so 'totally' converted. It is interesting that Kell seems to be schooling import pieces on his Benjuka plate against his Dunyain nature. Esme is important because she knows his true dunyain nature, he entrusts the Empire with her (I suspect because he knows a dunyain (Maitha) must fail against the WLW). Akka too. Now Proyas is being shown the truth as well.
It seems that because we no longer have a window into Kellhus' machinations we have kind of forgotten his level of Xanatos Gambit mastery.