Yeah, it's honestly pretty fucked up what they do with Flich. He can't do magic, so they have him clean up after a bunch of kids who can do magic better than he can already, even though there's fucking magic that could do that. He does more than just cleaning I guess, but that is part of it.
Probably not, just got more and more bitter with every year seeing kids do what he never could. Dumbledore thought he was doing him a favour by letting him stay at Hogwarts, but it must have been hell for Filch. On the other hand, he could have always just quit of course.