Well, after STBXH dropped the kids off at my work I drove to his parents place, taking an alternate route so he didn't know where I was going.
Had a tearful talk with his Mom and Dad, both of whom think I should put STBXH out of my mind. I agree. They're pretty upset with him embarking on a path of self destruction again and upset that he's lied to them about where he's been staying. Fortunately I didn't have to tell them.. they figured it out on their own when they contacted STBXH on the phone one (he told them he was sleeping on a male friend's couch) and they heard K's voice in the background. Also said they are catching him in other lies. In many ways, they are recognizing that it's like the last time. (In 2008 with OW#1)
My INs don't think there is a relationship to be salvaged and that their son will never want to be tied down. They said they had a talk with him about "harvesting his balls" if he neglected his financial responsibilities to his kids.
And they offered me their love and support. I'm very lucky indeed.