Well, I think you saw insanity set it right at the very end, after Merlin had placed the Curse of Babel on them, and everything was crashing down. I also think it's difficult to draw lines and precise definitions, because we don't truly and completely understand what possession is. There's a human will, there's an alien will, there's some kind of giving over and/or taking over, but beyond that, we're lost. Like the incident with the pigs at Gadarenes - the demons asked not to be "destroyed" (whatever that meant), but to be sent into the pigs - but then the pigs drowned. Did that effect the "destruction" of the demons, or did something else happen? We just don't know.
I think with That Hideous Strength, at least Frost and Wither had already been largely "devoured" by the demons in the same manner they wanted to "devour" Mark in that chilling scene that Lewis writes. However, it served the demons to let them have a reasonable amount of free will to be effective. Only at the end when they were no longer useful was their will overruled. And even that wasn't a total takeover, since there was still a vestige of choice remaining, even to the end. Remember how Frost was offered repentance even at the very end - but couldn't be bothered with it?
I think with That Hideous Strength, at least Frost and Wither had already been largely "devoured" by the demons in the same manner they wanted to "devour" Mark in that chilling scene that Lewis writes. However, it served the demons to let them have a reasonable amount of free will to be effective. Only at the end when they were no longer useful was their will overruled. And even that wasn't a total takeover, since there was still a vestige of choice remaining, even to the end. Remember how Frost was offered repentance even at the very end - but couldn't be bothered with it?