You think your 13 yr old doesn't have a clue what is going on with his mother? I mean with the drugs and the A, not about the townhouse.
I am not suggesting you do something to damage her even more in the eyes of her children, although, what could you do worse than what she has already done to herself? I merly said you should not trust her to give her "version" of the story and you should be present when they are told. The fact she is insistent on telling them alone, should give you a hint that she is going to gloss it over. I don't mean they have to know the gory details, but neither should they (especially the teenager) believe part of this your fault.
I think you should be scared! Your children are exposed and vulnerable to a number of bad things, and you know for certain part is connected to this man. She is on drugs and you have not considered he may be supplying? How could she buy it, of you are having to give her financial help? She is brainwashed, manipulated, and apparently controlled by him. Whether it's through drugs, psychologically, or both......he has remained in control. And, she is an adult, so what chances would a child stand? She said you didn't understand or it was complicated. That is coming from a woman who is not in love, but being controlled.
I have no doubt you love your children with all your being. I just have a hard time understanding why something can't be done to ensure their safety, considering the situation. If their own grandmother is ready to do this guy in.....that should tell you he is really bad news. Why not get a restraining order, where this man can not be around them?
I can't remember, but don't you have physical custody of the kids? Why are you having to pay her child support? Did this actually go through the legal system, or you just give it to her to help out?
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!