Hey Artaois,
in the Great Ordeal there is a scene with Kellhus being in Hell, a head on a pole behind him, and apparently talking to a ciphrang (Cnauir according to my suspicions) who tells Kellhus he has been here before. If I am guessing correctly, the Outside is privy to past, present and future, meaning the denizens of the Outside, including Gods, Ciphrang and whoever else is there, can see all events. So there might be a time paradox with Kellhus's soul, if you recall in TGO Kellhus sees himself in his dreams without the haloes. That Kellhus without the haloes could be future Kellhus, there is that wacky theory.
As for Kelmomas, there is this theory that he is twin-souled and Bakker has said in the Q&A that the crux of the issue was the lack of identity of Kel if I remember correctly.
If you recall, at the end, Ajokli possessed Cnaiur and was angry that Kellhus cheated him somehow and could not find him in Hell (because if he did, he would have a hell of a time). There is a theory in this forum that the dream Kellhus has with the tree and his version without the haloes might be his sanctuary from Hell, we will see about this one.
I think that Kellhus prepared contigencies for being possessed by Ajokli and Kelmomas interfering.
We were shown Kelmomas from the eyes of a Scylvendi who died mainly because that is a writerly techniqe to show us some important action without getting in the heads of the important characters in that scene - in this case the skin spy as Esmenet and Kelmomas, plus the scene acting as a red herring because the boy there could have been Crabicus.
Hope this gives you some clarity.