For the first part, sure you could be right. Still, once the books are over and they become adults, they don't do anything about it. I think Hermione goes on to become the minister of magic or something, but she doesn't try to change the system. This is presumably because Rowling thinks wanting to chang the system is childish and wrong, but that's just a guess.

As for the latter, no the house elves don't all want to be slaves. We know Doby at least didn't want to be. Just because Rowling wrote in a character after the complaints came it that is an elf who's entire purpose is about being owned doesn't undermine that at least some don't. It also doesn't even begin to discuss the indoctrination behind this and if that's acceptable.

The hero children also recognize the mistreatment of other intelligent magical creatures is wrong too, such as the centaurs or goblins. Again, they are made to not address this after the war, and also almost no one in the society seems to care. These themes are everywhere in the books, yet any time anyone tries to change the system they're mocked.