No wonder you are hurt.
it all seems really odd. I think some women ARE threatened by their partner's mothers for some biological reason that I don't claim to understand.
And conversely, I think mothers of sons are biologically programmed to go a bit tigermama when their sons form a serious relationship, especially if that relationship takes the son away or means that access to the son is not as free and easy as it once was.
I am not suggesting for a second that you have done anything deliberately untoward regarding this woman, but I do wonder if you are subconsciously giving out signals to her which are causing her to react the way she does. Is it worth thinking about whether that might be the case?
Also, the stuff regarding SS is frankly barking mad, and I think that you made mistake by brushing it under the carpet, although I can understand why you did.
In terms of what to do next, whatever you do, think about what you want to achieve as your main objective.
I am guessing that your main aim is to keep your relationship with your son on good terms. I would try to block out her behaviour, her sulking, her absences from visits, the lack of cups of tea, and focus on trying to facilitate a loving friendly atmosphere between you and your DS. Again, while xmas would piss me off, I think it's an ancillary issue here. Money < your son.
Unless your DS has the skin of a rhino, he will be well aware that there is a major issue here, and for whatever reason he is not open to confronting it and discussing it with you right now. Until such a time comes that you are able to speak constructively about it with him, I would bite your tongue and just continue to be there for him as his mum. While this seems unfair, my worry is that if you step up your behaviour or employ tactics, you will be playing into his partner's hands. She perhaps hopes that she can just ignore you/ be difficult and one day you'll get fed up and stop visiting. Don't do that.