Your lack of faith in Kellhus is disturbing.
I'm firmly planting my flag in the camp of Kellhus still being the major player. Here's why.
1. Kellhus saw himself within the inverse fire, not as fodder, but descending as hunger.
2. Nauir was described as a Prince of Hell by the judging eye because some souls were too powerful.
3. All the gods exist outside of time in some fashion. So once you're a God, you've always been a God since even before the beginning of the series.
4. Nothing that has happened so far, contradicts TTT as outlined between Kellhus and his father Moe.
5. In the conversations between Khell and the unnamed outside entity, the outside entity says he wars with the God and to draw him out he needs to raze the fields. Khell says he is the one who tends the fields (paraphrasing here).
So all signs point to Kellhus being some demon. He's a hunger in the outside, not food. So that means he's *always* been around. I suspect he's the outside entity Kellhus has been communing with and has been using his mortal past self as a tool. Moe's version of TTT specifically outlined how premeditated disasters would keep piling on TGO. Kellhus sees farther but we're never actually told that he would reverse the disasters for TGO. I believe in his own Dunyain way, Kellhus, as an Outside entity, would do what he does and use anything and everything including himself. The endgoal is to take down the God.
I think of the meta clues of the series. If the last series is just the downfall of mankind, there's no narrative there. It's just losing and there's no point in extending the series. Since we know Bakker was going to end the series with AE, the closest way to end the series while having leeway to extend it by a few books is a bootstrap paradox. Originally the series ends with Kellhus completing a temporal loop, becoming the God he's always been the entire series. The extended series draws this process out.