I am sorry you are going through all this. It's a shame your 'family' don't have as strong sense of kinship as you do.
It sounds very very toxic, what worries me is if you guys have kids. You say you don't like your mothers parenting skills, but who will want to get involved when you two have your own kids? Your mother. How will your partner feel with the sudden influx of family interference that comes when you have their grandchildren?
I can't imagine much good coming from that scenario.
It's sounds like your partner is constantly under stress that at any moment something will set it all off again, and you'll be back at the butt end of the abuse. Also you don't mention if he has any family support of his own, so I am going to guess not. Does he have a good group of friends he can go to for support?
They say we can choose our friends but not our family, you need to look at the long term and what is realistic over the long term.

@Bill3
The trump thing might have been immature, but turning up at her work screaming at her is hardly the picture of maturity either.

All the best for your future, and I hope that everything works out for you.