You are trying to make sense out of something that simply doesn’t.
If you are concerned about your son’s safety, address it through the courts. It is the only way that she MAY hear your concerns.
This could be MLC, but I feel like you think it is bigger. That there are drugs involved. If there are, especially hard drugs, which is what I think you are hinting at, there will be much more confusion that simply the usual MLC stuff.
It is ok to be angry, but please do your best to keep your son out of the middle of that.
You cannot make her be a good mother right now. That is up to her. Just be the best father you can and give that little guy the best of his father. He will remember it.
"Acceptance doesn't mean resignation. It means understanding that something is what it is and there's got to be a way through it."--Michael J. Fox