Mighty, I'm so sorry S17 is giving you a hard time. Clearly, he is having his own hard time.
Xh should not be allowed to scoop S17 up like that. It is completely disrespectful to you, and it teaches a TERRIBLY MISGUIDED lesson to S17. Are there custodial orders that are being violated? I can't remember your agreement. Either way, it's not good for S17 to have that kind of power to be able to call other parent out of rebellion.
I really hope S17 has a healthy outlet for his anger and other feelings. Is he in counseling? Imagine what that must be like for him. He's in HS, a senior...supposed to be the best year...and BAM. Betrayal, lies,humiliation. Same nuke as you, only he doesn't have the maturity and experience of an adult, to sort it out. That has to make him spin. Where does he go with his feelings? Just being a senior and having the pressures of entering adulthood, that alone is stressful. Add to that, your S17 is "half" made of this guy who did these horrible things...that in itself causes confusion.
"If dad did xyz....and I didn't know....did I ever really know dad? Do I love him if I don't know him? Do I hate him? Am I going to do what dad did when I'm M? If dad does bad things, is he bad? Does that make me bad? Should I start acting out and doing bad things? That's what bad people do, after all. So...Is my loyalty with mom or dad....because they are opposite sides. Do I have to pick one?"
No wonder he's losing it. Some self destructive behaviors, yeah?
One thing I would tell my kids about turning 18? Yep, they get to do what they want. Sure can!! Wooohooo!!! They're adults! The beauty of it is they also get to experience the real adult consequences of their actions. And it can be far worse than just being grounded for the weekend. And you can not call daddy to scoop you up in all circumstances, such as jail. House rules are like laws. Grounding is like training wheels for real life consequences. Xh is living real ones.
The manipulating and playing one parent against the other is very common. Here's the thing....it CAN'T pay off. I don't know the answer for your S17. What I do know is that it will keep being used as a tactic to get his way unless it doesn't work. You and H, ideally, would need to be on the same page we this. How possible is that? In MLC with no expectations? Idk.
I'm so glad we can vent here. Be the rock for S17. He'll come around.