I think you are doing the right thing by not going as well. Just because you don't go to a Thanksgiving dinner doesn't mean that you are abandoning your mother. You can still have a relationship with her but within the boundaries you set that don't allow the abusive BF to be a part of it. Explain to her that you don't feel comfortable with him and do not want to have your family around him. Make sure to let her know it in no way reflects your family's view on her. You wish to have her in your family's life just as much as she always has been. You and her simply have a different view on the BF and that is ok. She won't change your mind and you won't change hers as you are both adults.
If she gives you an ultimatum(which it seems is highly unlikely) that you either accept the BF or get out of her life, tell her you are sorry she is making such a harsh decision but you can not condone his behavior or subject your family to the possibility of being around it. Let her know you love her and your door is always open if she needs you. If that does happen, remember that you are not the one who closed the door on the relationship. You provided options and she refused them.
I wish you well my friend! I've got plenty of "family history" we'll call it that I can relate to your thoughts. Enjoy your Thanksgiving!
The truth is you're the weak and I am the tyranny of evil men. But I'm tryin Ringo...I'm tryin real hard to be the shepherd.