Here's my thing, there is actually no textual evidence that Kellhus isn't a real prophet. Maybe the God like him for trying to rid the world of Alien beings trying to fuck up his gig. Shame we never saw him with the eye, and we all know that was done on purpose.
Not to go way too far down this path, but its kind of like how a lot of people demand that either everything is done by God, or nothing is. In reality, if there was a god, its entirely possible we wouldn't be able to tell what was and wasn't directly done by it. Evolution, for example, could have been meticulously orchestrated by God - there's no reason for evolution to disprove its existence.
Similarly here, I think you're right in that Kellhus could be an avatar for some god that we don't know about. We're trying to prove a negative here (can't prove he isn't a prophet), and I don't find that line of reasoning particularly interesting as its functionally impossible to prove. Its along the same lines as claiming Moenghus Sr. is still controlling everything - yeah, the psuke could be used super secretly to do things we can't see or know about. Not wrong, but to me, uninteresting.
Kellhus isn't a real prophet because that's one of the main linchpins of the story - a normal guy falsely claims divinity to manipulate people to do his will. "No evidence" is also going a bit far. There's lots of evidence, its just disputable. Sure, we can ignore everything Kellhus says to the contrary, everything he does, the story structure and themes, we can ignore Psatma, Yatwer, Ajokli, the Dunyain, and everyone else that says he isn't a prophet, and come to the conclusion that he's actually the opposite of what all those things point to. We could, but I don't agree with it.