I've started writing this message and couple of times, trying to be sensitive but actually I'm just going to say it as I see it - STOP being 'KIND' ! You're not being kind you're being soft and allowing another person to invade your family and in the short, medium and long term getting yourself enmeshed.Just stop
Coming around when your DS is there of an evening, staying in his room, or another room away fom the rest of the family, sharing the occasional meal is fine. But from the sound of it - she's moved in bar sleeping there overnight. Inserting herself into your family - this is not on.
So my suggestions
Head your DH off at the pass - explain to him you want her there less and also want his backup even if it just to say when questioned by your DS "It's what your Mum wants and so that's what will happen"
Explain to DS that she seems to becoming a permanant fixture in the home and this isn't on. She's welcome 2 evenings a week and she leaves by 9pm. And not at all during the day - you're WFH and you need time with your other children when they come home from school, alone, just immediate family.
Next time she butts in when you're talking to anyone else - say "I wasn't talking to you". Yes it's abrupt but she is being rude and breaking commonplace politeness boundaries.
This is all going to put you on edge but you need to do this work to get your home back. As I say she's moved in bar the toothbrush, which will be the next step I wager good money on. She's already established a pattern of behaviour so there will be discomfort in breaking that cycle. Pain for you, your DS and her. But it needs to happen to defend yous home, your safe place.
Time to put on those big girl knickers User