Your mother has her views of the world and her views of you and what you should do. You are an adult now, and you have your own sense of yourself and your own understanding of what is good for you, what you can cope with, and where your boundaries are. Those two views won't necessarily overlap.
I think you are best listening to your own feelings on this and doing what you feel is in your best interests. I suspect it will be hard for you not to at least try and compromise with your mother's wishes - you're used to listening to her, taking her advice on board, and you probably want to please her. And you assume that - because she knows and loves you - she wouldn't suggest a particular course of action without having taken on board the impact on you and what would be best for you in the long run.
However, people aren't always that sensible. They get caught up with pre-set patterns, with quite unreflective ideas they've inherited and never questioned, and with their own - often unexamined - dramas and conflicts.
I think this might well be one situation where your mother isn't thinking quite as clearly as she might. I'm not sure that pushing you into a situation - drawing on feelings of guilt and obligation - where you are trying to care for a parent with mental health and addiction issues, which no-one (including your father himself) has managed to get a handle on for many, many years, is at all good for you. In fact, when you think of it in terms of a situation that no-one else has managed successfully over many, many years, you can see how damaging (and almost certainly futile) it's going to be.
The situation sounds very damaged, damaging and entrenched. You are very unlikely to be the person that can 'save' or help him. Which means that any contact you have with him will be on those terms: being there whilst he continues a pattern of destructive behaviour. Any contact you thought about having him would have to be done with that clearly in view.
It seems pretty clear that your mother hasn't quite reached the point where she can see that. I suspect she's not at the point where she can see that what she is suggesting is actually urging her daughter into a very draining, destructive, damaging situation.
It's probably very sad that your mother can't see that yet.
Perhaps it's fear - she worries that she feels compelled to 'help' and can't see how she can do that by herself. Perhaps she worries that you might feel guilt later, if you don't have some sort of contact now.
If the situation is as bad as you say, it would probably end in some form of NC or low contact anyway. Personally, I think you should heed your boundaries but try and talk to your mother about why she is pushing you to have contact with your father. If that looks as though it's going to end in conflict with your mother, though, stop discussing it?
It sounds like a horrible situation.