The mother may have had some trauma, something horrible is happening, which is causing her to act out of proportion to what is going on.
It may be something that went on 30 years ago which is causing her great anger at what supposedly happened 5 minutes ago.
There is no connection at what just happened and the amount of her anger, but she wants you to think there is. At 19, you might buy into that, but you probably shouldn't.
She also possibly sees you as a threat to take away her 22 year old "baby." Prior to you, she had no competition for the attention of her son.
Or, all of his previous girlfriends have also experienced her wrath. I had a mother who was a very controlling, also, yeah, and she had trauma, so I've seen my mother act irrationally.
It's hard to disengage from your mother at 22, or at any age, because they can cause so much trouble. For instance, what if you needed $100 bucks to fix your car so you could keep working? Maybe if you had let your mother rant at a few people, she might have given it to you.
I could not totally disengage. Now, I don't think I would allow her to be disrespectful to my girlfriend, but where do you draw the line? "Uh, mom, if you holler at her one more time and..." and what?
I've disengaged from close relatives, it's easy, but not my mom. Not anybody who might help me out with a loan sometime. It is tough.
Also, if she is so unstable that it sent her son to mental hospital, shows how irrational she is and that nobody is safe.
Maybe her son has to go back into her house, but maybe you don't. Of course if she's already called your grandmother and went off on her, she's showing she's undefeatable.
She can be dealt with. I would consider not going back in that house. And if she harasses your grandmother, call the police.
She's already put your boyfriend in the mental hospital. You'll be next if you keep going in there.
These are all my opinions. You'll have to do what you think is best.