Onit, that was my concern also. That he's been to or is going to a lawyer re Christmas plus whatever else he can think of. And I hope you're lawyer is correct that he'll come down to earth with a bang.
I was wondering earlier on if you could arrange to meet your MIL not at home initially as I think there is nothing to stop her, or you, from straying into dangerous territory talk wise and it could all go pear shaped. I thought I was being melodramatic so decided against saying it but would you be able to meet her at a cafe before picking up the kids and going home. Or even just meet the kids then go home. I think it makes for a safer 'not going there chat' because you'd be worried about people hearing.
Do you feel strong enough to just repeatedly say we are having Christmas Day at home, or I can't discuss Christmas Day - and all the more so because you don't know what LCB is up to with his silence. I'm just concerned that your very understandable emotions re Christmas and family are going to back fire on you because of initiating contact with your MIL. What would you be doing right now about it all if you hadn't met up with your SIL a few days back because it's that meeting that's prompted all of this.?
Re your husband being insecure. He probably is but I suspect his doggedness is more down to losing control of you than anything else. This is one very big power game for him.
Apart from organizing the birthday party and the other things you have to do - do you have anything planned just for you? Have you ever gone to the cinema alone for e.g.? I do it a lot. I love it. It's one of my very simple pleasures in life. Would you fancy giving it a try?