If you google "Love and Stockholm Syndrome", you will find articles related to the psychological control batterers have over their partners, and it may help you understand your mother's defense of her partner.
My opinion is that in defending her partner, your mother abandons you. If you decide to place strong limits on how much time (if any) you spend with your mother or allow her to see your child, try not to feel guilty for abandoning her. She has, I think, already abandoned you.
I would never ever underestimate the violence her partner is capable of, and I would never again be in his presence under any circumstances.
If you would like help in sorting out making good boundaries to protect yourself and your child, you could call the Domestic Violence hotline at 1-800-799-SAFE and ask for some guidance. It would be best to have some advice from experts about how to protect yourself and your child. And you can suggest to your mother she can also receive help on this hotline, though she may never decide to make the call.
I know this must be terribly painful for you. It hurts when people we love are in destructive relationship, whether with drugs or with abusers. But in recovery, we come to accept that we are powerless over other people. And we learn to make the best choices we can make for our highest good.