Astarion is constantly dissapproving of me - really constantly. But he is still in the neutral range. I don't think, it is possible to get a companion so low in approval, that they leave? Probably with the exception of Wyll, who - as I read here, never did that - will leave when you side with the goblins.
No you can, but it's really very difficult. You need to literally play against him and take every opportunity to do so.
