I hear you about NPD being a medical diagnosis/disorder. I believe my N learned to treat people the way her father did, because you're a product of byour environment. Now in no way is that an excuse.
With years of therapy I learned this isn't the way you treat people.
I learned to let go of the anger, the hatred, the fear because even though I'd be NC for years that that still gave her power. I don't want her to have power over me.
I remember being mortified when she got a job as an operator at the place where I was receiving help for my mental illness (this meant I heard her voice several times a day over the PA system) that day I freaked! I mean freaked.
But I knew she didn't know I was receiving services there. They locked my file away and hid me in the system as a precaution and also the save face probably. There was nothing they could do legally to fire her. They didn't know who she was before hiring her. Even in all that I had to remember not to give her power. I got over it, I stopped going to the day program for treatment so I wouldn't have to hear her page people. That wasn't a loss for me since I had pretty much graduated from the program, so I suppose it was nice timing.
I say don't forgive these things, they are unforgivable. I think it's ok to just let it go. We can never truly make people do what we want or need, thanks to free will. There will probably never be any retribution for any of the things that happened to us as kids. But we can learn how to treat people like we want to be treated and go from there.
Maybe you can let go of the guilt because I don't think you ever have to forgive them. People are going to do what they want to anyway, and the only person you really have to live with is yourself.
I don't know if this will help at all, but I hope you and everyone here will find the peace you need.
Please take care of yourself.