I've actually had thoughts along similar lines, Som, particularly that he is not one of the Few in the traditional sense. My personal idea is that Dunyain training creates an exceptional ability to honestly perceive the environment, and that once they are sufficiently informed about the nature of the Onta, they are capable of apprehending it.
Great explanation! I agree. Just as Kel observes and reacts in accordance with the world-born, he would have observed and calculated that the onta truly does exist, thus allowing him to manipulate it like a sorcerer.
I find this charming! The order of events seems to leave some wiggle room: Kellhus is bred/trained to overcome all circumstances, Kellhus encounters something he can't deal with (Mek), Kellhus adapts his shit until he has the Gift of the Few, Kellhus meets Akka
Would be neat if he could go even further: he needs to sorcery AND be impervious to chorae, so he finds an adaptation, assassin thinks they're going to kill him in TUC, Kellhus just grabs the chorae and eats it to show off, spits it up and makes it explode in assassin's skull.