I don't think establishing boundaries is gonna cut it here. Hime is used to getting her own way to point that she wouldn't hesitate to incite an epidemic or a massacre. There is no rational way to deal with a person like that. That fact that she wasn't put into isolation and had her charm skill confiscated or sealed speaks to how poorly this was handled. Even if she was reduced to a commoner she still had her skills, contact with allies and a low amount of security. Not the FL's fault.
Sure, but if she used her powers to bulldoze everyone that got in her way or she didn't like, that would make her a tyrant and no better than Hime. Also, she has power but no stable support. The public is fickle and the aristocracy has been of no help whatsoever. The allies she does have are in just as precarious a situation. So how does she win in a defensive position and not compromise her beliefs?
She should have put boundaries up with Hime in their previous world. The fact that she keeps being passive must mean she doesn’t want to live at this point.