I believe David (Kirk's son) changes as well, which would cross him off the IC list (unless we want to say the story is broken).
I think one of the reasons this installment is considered so good is because it seems like there's a lot going on in it. Maybe there isn't really, and we're just spinning our wheels, but there seems to be. Life, death, resurrection, the road not taken, revenge... it could all be an excuse for lots of shooty lasers and stuff blowing up.
I think the IC has to be either Spock or Khan. The Marcuses are only a device to shed light on Kirk's throughline. They don't even play a role beyond being the generic scientists being attacked until half the film has passed.