Hey, welcome to the group, and congratulations on getting married! Your H sounds like a great guy.
This is really positive to hear:
When she found out I am not taking their name she had a complete meltdown, disowned my husband yet again, and began texting him with a litany of illogical complaints about me and my family, telling him that she is sure we will get divorced and she is only trying to help him "see the light". My husband responded and told her that the decision about my name is between us, has nothing to do with her, and that our decision is final. He also told her her behavior is inappropriate and will not be tolerated. She responded with an "I'm sorry you were offended BUT... (insert laundry list of complaints) text and then demanded that we go to their house this week to explain my decision to her.
It sounds like you and your H are on the same page about his mom -- that's big. So glad you two can be a united front.
Does it feel like not going to their house to "explain yourself" somehow equates to "the start of no contact"?
Sometimes, pwBPD are very good at setting up situations that feel like lose-lose for you, but win-win for them. Maybe this is one of them -- does it feel like "either you go like she demands, or she'll never speak to you again"?
What if there's another way forward? One thought is to not respond to her demand. Not say "No we won't", not say "Yes we're coming", just... not give that message any engagement.
It's a negative, demanding, controlling message, and to respond to it at all -- even to say "this is unacceptable" -- may give her "negative engagement" (that is to say, proof that if she says awful things, her son will still engage with her).
Maybe you aren't required to respond at all.
What do you think?