Two points. First, about choosing Griffith over Guts, I'll point out the relative situation each was in. Griffith needed someone to take care of him at that time. Even though she cared more for Guts, she felt obligated to help Griffith, she wasn't willing to abandon him when she thought he needed her that desperately to take care of him. Remember, before she knew that Griffith was crippled, she'd agreed to leave with Guts, essentially choosing Guts over Griffith. It was obligation that made her stay with Griffith, not loving him more. Second, the fact that the rape snapped her mind at the time doesn't mean that she won't be able to deal with it as a memory. No matter how powerful a memory is, it's never as intense and crushing as the actual event, and she's very strong. I think she can cope.and remember casca may be a strong woman, but she already has shown she couldnt handle the trauma in the first place, and she already once made the decision to leave guts (for griffith no less;)) so its not like she'll follow him to the ends of the earth.
I seem to recall him explicitly stating that he'd never leave her behind again, which would seem to contradict that. Where could he possibly leave her and know she'd be safe? There isn't any such place, not with Griffith in the real world now. As for not being able to fight with her around, he's fought a number of Apostles since she's with him, having her sane would have to make it easier. I don't see why he wouldn't be able to fight them just because she's around.Also it doesnt really matter what she does, Guts wants to get rid of her anyway he cant fight apostles carrying her around.