I think it's pretty plausible that the request to bury him would certainly be suspicious post-conviction, but considering that we know nothing of how she was found guilty, we really don't know what they would have thought about then finding his tomb empty.
Sure, we don't know the circumstances of Iëva being found guilty, but I think it's plausible
someone wondered about the very specific request to have him buried after her trial/execution. It's not like she would have claimed that
because she intended to kill him - after all, cremation would have destroyed all evidence of poison while burial would not. I feel that it's likely that someone noticed something did not add up regarding this supposed "request", even if not immediately after the fact.
The entry, in stating that she was found guilty of his murder, at least on the face of things, implies that the popular conception was that he was indeed dead. We simply don't know if she was just found guilty, or her connection to the Consult was revealed. The latter seems unlikely, because if it was, presumably Seswatha, et al, would have viewed this as a bigger deal than it (seemingly) was treated as. Then again, we just don't know. There is a chance though, that this was why Seswatha was so dismissive of Celmomas' declarations, knowing that Nau was well in the Consult's clutches and outside any help. What could he have really done with the knowledge of the Consult having taken Nau? And could he have really fathomed that Nau was now the No-God? Even if he did, was there really a different course of action to be taken?
I didn't claim that Iëva's connection to the Consult would have been suspected. I actually don't think that was the case, at least not right away. Most likely, everyone just assumed she murdered her husband simply out of jealousy.
I do agree with you that the
majority of people, including Celmomas, had no doubt that Nau-Cayûti was dead. But it wouldn't be out of character for Seswatha, if he discovered the truth of part of it, to not tell anyone about it. After all, a) he had no evidence even if he did have suspicions, b) like you said, it's not like he could do much about it if it was true, and c) even if he was sure Nau-Cayûti had been captured by the Consult, best to let everyone keep thinking he was dead than to reveal his actual fate had been far worse.
That's what I was trying to get at when mentioning Celmomas' death and his supposed vision of Nau-Cayûti (actually Kellhus, wasn't it?). Could be just Seswatha thinking Celmomas was hallucinating, but it could be more.
I know it's a stretch to think Seswatha could have realized that Nau-Cayûti had been placed in the Carapace and not just slowly tortured to death by the Consult. But I believe Seswatha knew far more (not necessarily about this subject) than we
and the people in-universe think, so it would be an interesting possibility (and a tragic one, like I mentioned in my previous post).