I may be in a minority here, but I do see a positive thing to build on: the fact that your MIL offered to pay for you to stay in a motel. If you can find one that is not sensorily challenging, then I think this could actually be a really good compromise and a way forward. Family visits can be challenging. I am one of five children and one of my brothers (who I now think has undiagnosed sensory issues) has a VERY hard time at family visits because with all of us home it is complete mayhem. He's very inflexible and needs his routine and it is just torture for him. My parents pay for him to stay at a local motel at family Christmasses and it is a huge help to everyone: it's a form of respecting his needs, not a way to isolate or exclude him, and it makes it much easier for him to participate in the family event on his own terms. He is in his early thirties, and there are five of us between 38 and 22 as well as five grandkids ranging from nine to two. My brother shows up for lunch and part of each afternoon and he is GREAT company when he is able to visit on terms that are comfortable for him.
The biggest obstacle to taking up this offer, of course, would be that dh rejected it and seems uncomfortable with it. It sounds as though your dh is still in some denial, and in mourning for the life he might have if he had the child he was expecting. He still wants to think that things could be the way he imagined them. It's presumptuous of me to claim to know how he feels on the basis of reading two threads, of course, and I'm sure you have better insight. Maybe you could tell him the story about my brother and suggest that this is actually a really kind and generous offer from his mother? Then you get to be not the bad guy too: you're a bridge builder rather than someone creating division. I'm sure it's not just the dog, either, for your son at your MIL's house. You might have an easier time maintaining his routines at a motel: then you could meet up with your MIL for the parts of the day when he is at his best and have a really nice family visit with less stress for everyone. Of course your dh could also head over and hang out with your MIL at other times, and even if you're worried that she might sway him or they might gang up on you, it will probably be really good for them to have some mother/son time, and it might help your MIL in understanding what is going on in your family.
I hope my perspective is some help, even if you don't like my suggestion!