I would forget about N-400 for now. You have an issue getting your I-751 approved. Without I-751 approved, N-400 is worthless and won't be approved.
Battery / extreme cruelty waiver sounds like a good idea until it's not. Many think it's an easier route to paint the other party as a bad guy. What you describe is unfortunate and sign of toxic, disfunctional relationship. I'm sorry you had to go through all of this. At the same time, it does not seem to be a strong enough case for USCIS.
Did you seek shelter at any point? Did you have any witnesses of you being abused? Did you now get mental evaluation?
Good luck