I really think you're overestimating how sympathetic Nu is; I get that her life sucks, but she's still insane to the letter. Even after Ragna "saved" her, she still stabbed him in the back and very much wants to merge with him, it kind of established that she's a danger to everyone around her no matter the circumstance, that's something you have to consider when addressing her and it wouldn't be out there to assume the writers put her down for good in the next game.
Ragna isn't the reason she's the way she is, that's just how she's programmed along with having Saya's memories; she's compelled to merge with the other half of the grimoire and has these fond memories of Ragna that she probably doesn't understand, so it kinda made her go insane.