Am I wrong in thinking that he might be having a slowly developing mental break, or is developing some sort of condition akin to dementia, but, you know, with these symptoms? Especially if OOP thinks he just slowly changed into a completely different, obviously unwell, person and this is otherwise completely out of character?

Him slowly idolizing more and more those whack jobs is a pretty clear sign he’s either joined a cult and is being brainwashed, or he needs help, badly - but if he won’t accept help, not much you can do until he becomes dangerous to others or himself. Had something similar happen to an old friends brother - completely normal dude who turned into a Nazi sympathizer and Q follower over about 6 months. Took about 2 years, during which he almost ruined his life for good, but he finally accepted getting help and found he had schizophrenia. He’s still not the same, bubbly guy he was before, but at least he’s done with that nonsense and has an opportunity at life again