I don't think Kellhus knew that Ajokli was going to subsume him - though, I am a clear minority on this point
I'm with you here. Just finished my first read today and after the shock wore off I re-read CH 19/20 again. I suspect Kellhus was actually intending to stop the Consult and save the world. Being so close to the Inverse Fire gave Ajokli enough strength to break whatever hold Kellhus had on him (Metadaimos, anyone?) similar to Kakiol and Iyokus.
The question becomes: Why was Ajokli involved at all in the first place? A few ideas...
1) Was Kellhus wearing the Trickster's head on his girdle as a way to prove to the gods that the Consult and No-God exist? If it really is Ajokli speaking the entire time in the Golden Room, it means he was calling himself the Inverse Prophet, which makes sense... a God sent into the world of the Living to report back to the Gods, "Yeah guys Mog-Pharau is a real thing". I don't think this is the case, however, since the Hundred are clearly hunting Ajokli.
2) Ajokli possessed Kellhus while he was staring into the Inverse Fire. I'm simply listing this one for the sake of writing down anything that comes to mind. I'm pretty certain this isn't the case, considering we see Kellhus using Ajokli's powers as he gets closer to the Ark (during the Last Whelming to float, for instance).
3) Kellhus was possessed when he ventured into The Outside, and Ajokli simply continued with Kellhus' plans in order to get to the Golden Room and obtain full power / take over the world.
4) Kellhus has been an evil bastard the entire time and worked with Ajokli willingly, knowing that getting him to the Golden Room would grant him god-status.
Gah, I don't know... there are so many things to consider! Why/how did Ajokli inhabit Cnaiur at the end? Why is it that Kellhus can see Kel, but Ajokli-Kellhus can't?
It's going to be a long wait for whatever comes next.
PS: This is a bit off-topic for the thread I think, but it's tangential... how did Kellhus know about the Mutilated "After Dagliash"? What's the connection there? Maybe it helps shine a light on the rest.