What you've been through with your mom must have been terrible, and going NC isn't easy, nor is keeping it up for two years. This is an impressive accomplishment. It's sad, absolutely, but I've been NC with each of my parents at times and while it hurts, it's for the better and I've come out stronger for it.

I know you were counting on your dad's support. It's clearly angering you to be so disappointed by him. I don't know how his being in touch with her makes it worse for you specifically but I'm not there to observe your particular dynamic and I'm not doubting in any way that what you say is true. And his actions must be frightening and hurtful.

I want you to note that you have transferred the control over your life from your N to one of her E's and back to your N. Your mom used to be the center of your world because her Narcissism demanded it and because she was in a position to abusively satisfy her narcissistic need. Now, your Dad controls how you're feeling about your mom: if dad doesn't do what you want, you'll lash out and cut cords with him too. Giving him an ultimatum on how he defines his relationship with your mom or you'll end your relationship with him means your mom is still in control of the relationships in your life. Your mom is controlling who can and can't talk to you and how you're going to talk to them...she's just doing it by a proxy she has installed in you.

Get out your uninstallation app. Undownload that shit. If your dad truly is her enabler and has indeed done all the things you're saying, put him on NC, too. You already know what that's like: it sucks. It's not easy. It's sad, absolutely, and while it hurts, it's for the better and you'll come out stronger for it.

But, if you're just angry because he has a relationship with your N? I'm not totally sure that's grounds for going NC unless he abuses his relationships between you intentionally irresponsibly on a regular basis.