I think you've messed up. He cheated on her and he was violent to her - and probably a whole bunch of awful things you don't know about because you're her child and she did her best to shield you. As far as she knew, this douchebag was out of her life forever, and out of yours - and you just casually mentioned you now have a relationship with him.
You should have told her 7 years you were trying to build a relationship with him, not now. And not by just 'mentioning' it. This is a much bigger deal than you originally suggested in your opening post. She's had no time to get her head around the last 7 years of you deliberately lying to her by omission.
She's your mum, so she'll forgive you, but wow, the poor woman must be in absolute bits over this. Really, really thoughtless behaviour.
I think you should be less concerned about your mum ruining your big day by being miserable, and be concerned at how much you've hurt her. I also don't think that a child should necessarily have a relationship with both parents if one of them is abusive, and you don't know the full story. If someone was nice as pie to me and did something unspeakable to someone else, I wouldn't have them in my life. When selecting your inner circle, you need to look at how people treat everyone, not just how they treat you.
I don't mean to make you feel bad, but I do think you need to speak to your mum on neutral ground when you're both calm, and to have an honest discussion about what concerns she has about your dad being in your life (not just your wedding). Given she was always there for you, and the strength of her reaction to the news, I think you owe her that chat.