which feels like she's also trying to abuse me which is tough when I'm already stuck in a cycle with OH.
Why are you letting your OH abuse your children? You know he is, you acknowledge it here.
I’d be pretty furious that my mum had ignored my wishes, whatever they were; and encouraged my DS to lie to me… if she disagreed with something I’d expect her to talk to me about it, not do it anyway. But it’s quite possible here that she’s intervened in the only way she can, because you’re willing to put up with this. You’re condoning your partner setting a bizarre menu and expecting everyone to obey it. And even if she just did it because she wanted to get him chocolate, what she’s done is far; far less damaging than what he’s done.
Plus you’re not entirely sticking to it, if he’s got a normal advent calendar, which does undermine how seriously it’s being taken…
The more time your children spend around him, the more work they’re going to have to do when they’re older. The best thing you could give them this Christmas is being far away from him and his controlling influences before he decides that you’re all doing something else batshit and damaging.
You can’t expect the HV, school etc to step in if you seem to be condoning this. By the time they make referrals, they’ll be against you too, if you’re just passively waiting. If you need their help to do it, talk to your HV tomorrow, and Women’s Aid. You’ve got no chance of building a better support network before you leave; so you’re going to need to do it afterwards.