There are others here with more legal experience, but I just wanted to say that the situation sounds very serious. And doing nothing but waiting for a crisis does not seem like a good choice. If either of the adults are driving high, your children are in danger.
Was your custody arrangement made when the divorce was negotiated? That was a year before your exAH became involved with the drug addict/criminal?
I think you have reason enough to seek legal advice about re-negotiating the custody issues and visitation based on the father having a live-in girlfriend who has a criminal and drug history. It seems to me it would be possible to pursue a change in visitation requesting no overnights, for a start.
If you are dealing with two drug addicts--and it certainly seems likely--then you know by now that they will resent you anytime you take action they don't like, and will engage in major power struggles with you, and will lie and will try to inflame the situation. So I would not try in any way to work things out with them directly and I also would not try in any way to gather more evidence through the children.
I suggest you seek legal advice. Privately. And if your children do not have cell phones, perhaps consider providing those to the them, with some emergency numbers for contacts in case they need you or another trustworthy adult.
It is so awful to worry that one's children are not safe with the divorced parent. Others here may have better suggestions than mine.
But I do think your Higher power is trying to give you vital information, through the children and via your gut.
And, one more note. Getting your children into counseling could be also a help.