Such a shame about lockdown and the party as that would have been a nice pinch point for him to experience and decide which way to jump.
There is always going to be something ridiculous @workworkworkugh whilst your DS is entangled with this nutter and her enabling parents.
All you can do is continue to stand well clear so that all of the drama and nonsense is 100% generated by her and focused on her.
He will see it in time.
I think the “no-go” conversations are his volatility under pressure from her - he is trying to placate her all the time and it’s exhausting.....and he snaps at you (which is wrong) - request calmly and repeatedly that he doesn’t speak to you sharply. Don’t get triggered into snapping back. Just model assertive respectful request for him to lower the tone of his voice and speak to you with kinder words.
You can loath her all you want. That’s a very natural and healthy reaction to such a vile character - but let that out elsewhere - never with your DS or her as that will just backfire - summon up your power with deep breaths etc. Play GF bingo - you just know how each and every situation will play out - possessive, controlling, paranoid, difficult - be ready for it.
Little, gentle, reflections with your DS when you have the opportunity - never ask - so - how did that feel?
What did you want to do?
That sounds complicated?
Sounds like hard work.
Did you have fun?
Is that appropriate?
Were you happy with that?