So my interpretation is that Khellus never intended to become possessed by Ajokli. In fact it seems that the possession was progressing without his knowledge as he delved deeper in Ark. Bakker in his AMA said that the Dunyain spiritual strength is quite weak, leading me to believe that his work with the Daimos may have made him an unknowing conduit for Ajokli to enter the World.
Remember, the Gods are blind to the No-God. However, they see it’s shadow. Many of the Gods laid blame to Khellus for the madness (who else could be behind such maddess as the ordeal than the one leading them). This is why Yatwer was so intent on killing him.
However, it seems that Khellus somehow travelled to the Outside and became an Inverse Prophet, warning the Gods of the dangers that lay in the World. I need to re-read TGO because a lot of the head on a pole passages may be related to this interaction. I have a feeling that Khellus was hoping to be able to control enough of the Daimos to keep Ajokli at bay while using his powers for some reason. But he succumbed.
This may also be why Khellus couldn’t see the danger that was his son. The Gods live outside of time, so once Kel became the No-God, he was always the No-God, through all of time.
Bakker also stated for definite that Khellus’ goal was the stop the Second Apocalypse and the Resumption, thereby saving the world. He failed.