I don’t know how to help this going forward in a way where I’m not completely ostracising myself from him.
You're not ostracising yourself happyfarm. That's something someone else does to you. Him, in this case. And since it is his behaviour that means you can't control it, so there's nothing you can do except have no boundaries and get treated like shit. You're drawing a reasonable boundary. He's punishing you with silent treatment, it's emotional abuse.
I honestly feel a bit sorry for your husband. It's not an easy situation to be in. You don't like his family, they don't like you, they won't do what you want, you won't do what they want and all he wants is the easiest route through.
When he comes on here for help with his relationship with his mother that's when I'll feel sorry for him. You're making this sound like it's equal between his mother and his wife. Happyfarm doesn't like MIL because MIL doesn't like her and treats her badly. All happyfarm wants is to be treated decently with basic respect and kindness. It's not unreasonable. Whereas his mother wants happyfarm to be ignored and othered by her whilst she's with them. Which is totally unreasonable. She's tried to find common ground and get along with his family, but MIL/SIL/GC bro aren't interested. If her husband chooses to side with his unreasonable mother against his reasonable wife, he doesn't deserve sympathy. Basic premise of marriage: your spouse comes first (except DC), especially when spouse is being reasonable and the other party isn't.
He needs to manage his boundaries with mummy
Agree binkie.
Problem is, husband doesn't have any happyfarm. Though he's fine to set plenty of unreasonable ones with you, expecting you to suck it up.
It feels abusive and dismissive and I don't like it.
That's because it is abusive and dismissive - and you shouldn't like that.
the price of no contact ripples into relationships that they cherish….
I've found that the price of having contact does too.
When you're gossiped about to them by the narc,
when they're gossiped about to you by the narc,
when there's a narc shit-stirring between you in the background,
when you can't have a conversation between you and them in the narcs presence because the narc will be very vocal with their opinions on what both of you are saying and so take over the conversation making it all about them,
when the others also have to be put on an information diet otherwise your private life will get back to the narc even though you haven't told them.
You can't have a normal relationship with the others whether you remain in contact with the narc or not.