Everyone's story is different (and I don't wanna delve to much into how one experiences rape) but I imagine that being raped, betrayed, and having all of your family being murdered by someone you've admired for decades [practically a hero figure to you] is bounds more traumatic than being harmed by a stranger. Fictional character or not, there's no way you can compare Casca's situation to the "norm" tragedy.
First off, no…. Guts is not sane. How many panels have we seen of Guts being tormented by his past tragedies (prior to and after the eclipse massacre). Guts doesn't even like people touching him. Please don't act as if Guts is stable. Jesus, Casca's family… her whole entire life.. just went up in flames in the most terrifying way. How do you expect any character from Berserk to recover from that? Do you think Guts wants to put Casca (the person that he fights day and night to protect even when she doesn't show any compassion back) through all of that? What loved one would do such a thing? Guts will do what's best, even if that means losing the love of Casca.
Your definition of sane is a little …
For you an insane(not sane) person is a person with trauma. For me an insane person is a person that can't live normally in society, and by normally I mean needing constantly help or being harmful to others without constant supervision.
In my definition Guts is a sane person, by yours he is just like Caska, insane.
When you're saying that Caska will get her memory back, the thing is that she never lost her memories. The fact that she seems to not remember anything has nothing to do with memory loss and is just a defense mechanism, just like it seems she doesn't know how to speak.
Because of her trauma, her brain "closed" some knowledge to protect her. Normally it would only be the bad memories that she doesn't want to remember, but the trauma was so big that she basically "closed" everything that she knew.
What I'm saying is that the Supposedly memory loss is part of her insanity, she is only insane because of the "Memory Loss", so to make her sane again she have to "Recover her memory", well maybe the Fairy King can selectively make what to make her "remember" or what not, but probably that wouldn't make any difference to her stability. Why? Because the small traumas(like fear of people touching her, like Guts) would still be there either she "remembered" it or not, given that traumas are of a unconscious nature and not conscious.
Basically, she became what she is now, because she couldn't accept what happened to her, so she ran away from her pain and hid everything deep inside her unconscious. To return to sanity she has to accept what happened and move forward.
I don't know if today in real life is possible to bring back a person with such a deep wound back to sanity, probably it isn't, but if it is, then it would be interesting to know how would that process be.