Yes, but I think Bakker's answer in regards to his plan failing outline why that would make sense.
Then the timing doesn't really make sense. Why wouldn't Ajokli-possessing-Kellhus just storm the Golden Room by his lonesome a day or two earlier, if he is in such a hurry to manifest in the physical world that Kellhus's well-being doesn't matter to him? Kellhus was sufficiently close to the Ark for a while (I can't really buy that some couple of miles make the difference), but only enters it during the climactic battle, said battle being planned from the get-go. Postulating that he entered the Ark because he suddenly lost control exactly then seems contrived.
I'm not sure where you are getting the assumptions that he was close enough previously. If anything, I would say we're shown and told that him getting close is precisely an important thing.
- We're told a couple times by Bakker in his Q&A that Kellhus' possession was 1. a gradual process and 2. got worse the closer he got to the Ark.
- Think on Kakaliol, the ciphrang we see a couple point of view entries on. It isn't until he's literally right next to the massive torpos of the Ark that he can slip his bindings to escape. He was right next to it, trapped. And only once nearly touching it.
So it certainly makes sense to me that Ajokli likely couldn't manifest as he did until they were in the golden room a bit.
I think the phrasing 'suddenly lost control' is off. He was losing control for a while. Bakker even outlined this, "He drifted into it, before finally being seized in the Golden Room." We're told he makes other mistakes because of his spiritual duress, namely missing that Esement would free Kelmomas. His thinking was compromised. Why he might have thought he walked the conditioned ground can certainly be attributed to flawed thinking whilst under divine influence, if not outright possession.
On the other hand, it makes perfect sense if we consider the whole Ajokli-as-a-weapon thing planned by him. Kellhus needs to battle full Dunyain. However good are his sorcerers, it's unreasonable to think they would be on par with the Dunsult. Even Serwa is much weaker than her father, which is evidenced by him using the Cant of Translocation over and over again while falling to successfully battle a flying enemy and her being able to use it once in a more or less calm circumstances and then require a prolonged rest. So Kellhus's engaging a God as his enforcer looks like a reasonable plan and explains his words about walking the Conditioned Ground in the Golden Room even without his Ordeal. Otherwise we need to attribute those words to Ajokli, and then they sound off. Not unexplainable, but off.
[EDIT] Another important thing that comes to mind is Kellhus using sorcery before his head gets engulfed in flames, but not during that time. Do we think that a God, even possessing a mortal, would be able to use sorcery? At the moment of entering the Golden Room the possession would need to be almost complete, so there is basically no Kellhus left, it's Ajokli uttering Metagnostic Cants.
It hardly makes perfect sense. Why is it unreasonable to think they could have take the Mutilated absent a God? Serwa, despite being lesser, is still an amazing witch. Kellhus is likely the best ever, Kayutas is an amazing warrior, and the host of Schools are quite capable. The Mutilated had no more nukes, we see no more lazer weapons, they have no magic, and Aurang and Mek die. Seems like they certainly could have been their match.
I think you're viewing the possession as a switch flipping. One moment Kellhus, the next Ajokli. but several times the implication is that it a gradual process, a 'drifting'. I see no issue with the idea of Kellhus having sorcery at his command until Ajokli truly takes over. Kellhus is merely the one using it until it happens, whilst under the divine influence.