I do agree that if she doesn't go he will think he's won and I like your suggestion of OP having a plan to challenge him if he starts again.
But he will think he's won anyway won't he?
I just don't think I could bear being there him walking around my Mum's home and gloating because he knows she complained and he knows everyone else supported him and not the OP
@Tungstern had some good points.
I think his initial remarks were a bit of a power trip... what can I get away with, and now he knows - quite a lot. Remembering that he did this out of everyone else's earshot, He sounds like a divide and rule personality.
TBH I'm really not sure what I would do in the OP's situation. Only you can judge this @Christmasinvite I'm so sorry your Mum's reaction was so disappointing. She acknowledges it was unacceptable but has chosen the turn a blind eye route, the one where you shut up we can all forget it and pretend everything's dandy.
Do you think its because your Mum is counting on the fact that you will be reined in and attend, so she doesn't really think you'll boycott the event? Maybe she sees you as an extension of herself and thinks its good manners to ignore it. How would she react if you didn't go? Maybe you not being there is the shock they all need? Would she accept it if you didn't go, or get into a huff. Maybe that would illustrate to her what the consequences are of ignoring his behaviour.
I don't think You are losing a normal, happy, family Christmas if you boycott it.
Attending would carry a lot of anxiety due to dreading it for weeks ahead.
Could you guage in advance if he's learnt his lesson and intends to behave himself so that it might be possible to attend?
or would you be potentially having to watch him turn on an all out charm offensive to convince them that it was all in your mind, with the family saying afterwards, "You see OP, he's ok really, it time you got over it and accept him like we have." Which in turn could make you feel more upset at the lack of support.
What you can be sure of that this guy is stuffed full of confidence in himself to be turning up to Chrismas Day to meet his new partners family and behaving like he did. He's not a nice person, a lame non apology via your cousin doesn't count. Unless you have nerves of steel, you could end up being seen as the problem causer of the piece. You didn't want to cause a scene last time. It depends if you think your DM would speak up for you this time or wants to continue to "let it lie" or even if they'd weigh in to defend him?
It would be easier if you were able to challenge him, one to one, and not in front of all his supporters.
Can you think of other plans that would be a better alternative?
As they are fond of saying on MN.. when someone shows you who they are believe them.
Hopefully he won't be able to stop his true character from showing and you can all wave him goodbye.
Sorry OP.. Mixed advice I know, but you are better placed to judge their likely reactions.