The Mum contacting you is upping the ante.
She is fishing for info or a reaction. Probably the DD writing or dictating the texts.
Drop the rope with her.
Don’t give her the reaction or info she wants.
It’s self serving - she needs your DS as a buffer to keep her DD calm.
A single thumbs up response to anything you can get away with after 24hrs.
Anything else non comital, vague and grey rock, detached.
“Not decided yet”
“Will let you know”
“No thanks”
“No, that doesn’t work for us”
Don’t get drawn into anything with the DM - as when the shit hits the fan with school she will be able to demonstrate that you were actively cooperating and complicit in this abusive relationship - rather than trying to be cold and neutral.
I understand that your DS is highly stressed again because GF (and prob her M&D) are on his back - offer him calm - don’t be heavy with him about the birthday otherwise he is in the middle of a tug of war - maybe get his friends to set something up so that it is led by them and he would look like a dick letting them down not participating and then you and your kids just join them at some point?
This birthday thing is going to escalate for her - DON’T get caught up in her histrionics - so the opposite of Michele Obama here - when she goes high (histrionics) you go low (indifference, cool, not bothered).
Know that your DS knows you have his back, that she is OOO and that he is just stressed in the middle of it. Trust that he will come to through and back to your place of cool and calm.