One thing that I think is important, but hasn't really been discussed (that I've seen) is the fact that Celmomas doesn't seem to know that Seswatha went with Nau into Golgotterath. Now, you could chalk this up to the idea that King Cel is just losing it as he dies, but I doubt it. We're specifically being deprived of information about that event (the journey into Golgotterath), and I personally think there's way more to it. We know Nau got pissed with Ses about being brought there, and the implication I got was that it was under false pretenses. I absolutely believe that Seswatha is playing a "bigger game" than everyone realizes. He wanted to get into Golgotterath for a very specific reason, and I don't think it was to save the chick Nau loved. He wanted something else.
I personally think Ses saw the Inverse Fire, and realized he was damned for being a sorcerer. Even so, he hated the Consult enough that he would never join them. He had a "better plan", at least in his own eyes.
Ultimately though, I think the important question is, why Nau-Cayuti? Why did Seswatha lead him into Golgotterath? Why would the Consult prize him so much as to use him for the basis of the No-God?