I have that MIL, by the way. And it went on with her publicly and viciously picking at me for a few years. She was so bad that others in the family engaged in it to. My BIL's ex-girlfriend of 10 years called me about a year after they broke up to apologize for her behavior--saying that she did it because I was the enemy and you were somewhat forced to take sides.
I really won't put on the internet the things that went on. I honestly have no clue how or why I stayed married through it (especially since we didn't have kids). My husband going there for dinner without me was not completely unheard of, either. But I got sucked into the bigger stuff. He'd often say that he wouldn't go if I didn't go and I knew that was manipulation but it was what it was. He knew I wouldn't start that war given the way things were; and he'd have been upset if we missed it. He didn't say it in terms of "It's okay--I don't want to go anyway" where he'd stand up for that decision to his family. He wanted to go and he was the consummate subservient child--he wouldn't stand up to them for anything. And during all of this, we lived a 20-minute ride away.
Even when it got back to being civil, they wouldn't come to our house. I'd invite them for dinner and they'd never come. My MIL would buy her produce at a store LITERALLY UP THE BLOCK from my house and when DH noted that she's there at least once/week and never stops by, she started shopping elsewhere... after nearly 15 years (at that point) of shopping at this place.
Oh yeah--they really do NOT like me.
Fourteen years later, it is WAY different. But as some people in this thread alluded to, this is about your DH's relationship with your MIL. My dh got to a point where he realized that his family lived under his roof and everyone else were relatives. To that end, he was okay cutting them off if they weren't respectful. And that's actually what it took. They missed most of the first 2 years of my son's life as a result.
They will never truly accept me as one of theirs, but we moved halfway across the country 2-1/2 years ago and by then, things had gotten good enough that they actually came and stayed in our home for a long weekend our first New Years--because I privately e-mailed my FIL and requested that they come because my then-7yo was sad seeing the family together for holidays (during skype at Thanksgiving). We have gone back each year and stayed with them, but this last fall was the end of it. Inasmuch as they are nothing like they were in how they talk to us or treat us (we're still definitely outsiders, but not enemies) they have a hard time with how we live and accommodating it for our kids. They try, but we live much too differently (this really becomes a TV problem--they are A. DDIC. TED. and my oldest gets aggressive with too much screen time). This last trip saw a pretty big blow out about it, but it was more my FIL freaking about how to manage it all rather than berating us or my child.
So there is HOPE, but it will have nothing to do with an apology from MIL as much as it has to do with your DH really coming to terms with who the priorities are in his life and how far he will go to protect them. Why does he need to go back without you? To be with people who would do something like that to you and your child? I mean, is he seeing your MIL or is he seeing the other extended relatives? And who is he staying with? This would all be really telling. If he's going back, staying with someone else, visiting with the extended family (where I'm thinking you'd be welcome?) and just stopping by to visit MIL once while there... okay, I get that. But if he's going back and staying with your inlaws and they're not having significant discussions about how things need to be (trying to make progress to having them in your life but with respect and boundaries)... that would be a problem for me.
Although we went to therapy, I honestly don't know how much that contributed to it getting better. But that's where I'd start.