My two kellics to the discussion: I don't agree with the suggestion of a DEM. Nearly everything was explained or hinted at in advance. Yeteven with all the hints, we still got surprised by the way of the ending.
How Kel got through the crowd:
I think TGO demonstrated fairly well how the Narindar observed by Kel moves unseen through crowds. It's not unreasonable to assume Kel picked up the skill as well.
How he got through the Ark, was probably the same way Serwa was going to use. Note that how when the young Ainoni with the Cindersword arrives at the gate of Skuthula, the gate is already open. When the Kayutas and the Mandate arrive, the gate is closed. I'm currently assuming either Kel passed through just before the young Ainoni arrived, or that we were deceived and that Kel was the Ainoni under a glammer. I mean: the guy killed off a Tall in a few sentences. But I'm more inclined to the prior option.
Concerning Kellhus's power:
We get multiple descriptions of Kellhus's Mark throughout TAE. I got the impression it got deeper and deeper, and more blasted throughout the series, based on the subtle difference in adjectives used in the PoV of other protagonists throughout the series. I've entertained for long the possibility that Kellhus was either having people (Zaudunyani) killed to become more powerful himself (souls basically being dinner) in the Outside, resulting in more powerful magic.
After TUC, I'm expecting him having to sacrifice the Ordeal as a whole, as damned and deceived as possible (remember the Meat), was the price for Ajokli's assistance (since there'd be more dinner for him). I'd even go as far as suggesting that the sacrifice of the whole "maxxed out damned" and deceived (!!) ordeal was a necessity for Ajokli to assist Kellhus, which is why we in my perception get the horrific passages of the Ordeal going bananas both in TGO and TUC.
Proyas is in that case indeed "saved" from being served as dinner to Ajokli by Kellhus, by being told about the deception, albeit out of necessity. The way we see Kellhus apologising to him in TUC in my opinion shows Kellhus's true (Dunyain vestigial) emotions in his love for Proyas.
The same I think goes for Esmenet: Through deduction based on Ajokli's perspective of time/ events, he sees what is going to happen in Momemn and he puts everything at risk to save her, unwittingly causing his endeavor to fail by bringing along Kelmomas.
I agree hints have ben given throughout TAE that Kellhus had mastered the Daimos. The whole head on a pole sequence, rumours throughout TJE from the "foreign" protagonists, and even through Malowebi in his first TUC PoV. We even know he's been to the Outside through the Head on a Pole passage in TGO. The fact that he's made a deal with one of the gods was, in my opinion, extremely probable.
And the biggest "damn, I should've seen that coming during the whole TAE" was the joke of the Celmomian Prophecy: Even the Dunyain didn't get that one. Kellhus could never become the No-God.