Agree it all sounds icky and dodgy.
Your 40th is your 40th and I would prioritise making it YOUR day and steering well clear of the drama.
I'd take a grey rock approach to the situation and put your peace of mind first rather than get too involved.
You're 100% right to feel upset but if group nastiness has set in it's going to upset you further to try to analyse or solve it.
Work out what you want to do, which is the nice holiday for you (I did similar and loved it). This is now set in stone.
Then like a pp said have a lunch or evening out (which you'll be doing with your lovely DH and family anyway) and let people know they can turn up if they want to. Drop the craft event plans.
Mention if they want to have the weekend away to just "carry on without me, and I'll look forward to catching up soon".
But don't bank on it or do any decorating or preparation or booking. If anyone asks for information just be factual and neutral.
If they let you know cool, if not no bother. It's up to you if your lovely DH wants to pay for their meal (I probably wouldn't).
This keeps the door open if anyone is nice and wants to turn up individually. Relegate them to friendly acquaintances.
Then do your own thing and just focus on yourself and your family.
Take the wind right out of anyone's sails.
Joan sounds like she is a ringleader who is enjoying the drama and I suspect others are just tagging along.
Some "nice" people can be absolute dicks if they think someone else is vulnerable and then they get a vicious pack mentality.... it's to do with social climbing, or competitive reasons, or all sorts of nonsense reasons!
I often steer clear of small intense groups for this reason.
Life is short, things change quickly. Even if friendship groups are great they often drift away for various random reasons!
Ultimately it's only you and your direct family you can rely on.
I'm a social over-analyser but I regret all the time I wasted trying to "solve" friendship situations with people rather than put energy into me and my own life goals!