Part of why Kassy's so powerful but isn't godlike is actually in her personality. She's got a lot of magic, a lot, and if she really used it without any scruples, she'd make a powerful supervillain. Or have a cult.

I think that's the hinging point in what can be done with mental powers. Kassy doesn't shove her way into people's minds because she doesn't want to. She doesn't want to hurt people, she doesn't want to trespass. She could be absolutely broken, but what keeps her from being a Villain Sue is her own conscience. That may be the crux that is between a viable character and one that's broken.