"Toying with the idea of asking my IC to do one final sit-down with x..."
No, don't. Stop that thought. Totally inappropriate. Totally intrusive. Stop trying to work out the problems in an R that doesn't exist.
Moreover, X is irrelevant here. You are revisiting your comfort zone of misery. You are escaping. Why? What's going on?
Do you feel too challenged by DS? Do you feel like a bad mother? Do you simply want to avoid the anger? What are you masking? Whatever it is, you certainly managed to avoid it with you nasty drug of choice.
As for how to interact with XH -- STICK TO THE BUSINESS OF CO-PARENTING. His request that you call him with ALL problems is inappropriate and unreasonable. Is he really going to invite you into his household every time there is a problem with one of the kids over there? I think not. He is not going to want you parenting in his house via teleconference. No reason he should be doing that in your house on a regular basis either.
Notice that there is also no reason that you couldn't sometimes ask for help via text. "XH, DS will not get dressed and go to school because his ROTC pants are too short. If you could give him a nudge, it would be appreciated."
THAT is the content relevant to co-parenting. All the stuff about your XM is irrelevant. It is about you and not your son. And that stuff about your XM is for you to work out on your own. It is not XH's job, nor his responsibility. XH is not the person who will work out those problems with you. He cannot. He will not. He should not. So, put your XM baggage in a box, take it to IC. The interaction with XH is about CO-PARENTING.
Talking with XH right now does not enhance your ability to CO-PARENT. So, no reason to do it.
Also, try to quit making every problem with DS about the D. It isn't. Your DS is a teenager. The causes for his behavior are what they are. No doubt, the D makes things harder for you both. But, you don't have a magic magnifying glass that lets you see which issues are due to the D. And you don't have carte blanche to beat XH about the head with D rants every time DS has issues. How would you like it if XH started complaining about your actions during the M that he didn't like when you were trying to work on helping DS in the present day. The past affects us, but we can't change it. So, rather than beat up XH regarding something neither of you can change, how about: "I'd really like to ask for your support sometimes when I think that two parental voices will make more of an impact than one. Would it be OK for me to call on you for help sometimes to back me up?"
That is co-parenting, but I'm not convinced it would help you at all. XH is being a butthead overall, so you might want to really think about whether you want that help. Imagine you HAD called him this morning: "XH, I'm trying to get out of here and DS is having a fit because his ROTC pants are too short and he's refusing to go to school." What do you REALLY think XH's reply would have been? Personally, I think he would have gone off and given you a nasty earful, blaming you, criticizing you, feeling superior, etc... I don't think he would have said, "Oh, please let me help. I'll be right over and take him to school myself."
Nope. I don't see that happening. He SAYS he would have done so now, but this just lets him feel like a good, present dad when he isn't one. So, boooo him.
You are a good mother. You handled DS just fine, you found a great solution. That is what matters.
So, in the end: "XH, thanks for your concern. I may take you up on your offer of help at some point in the future. It will be good for DS to be able to benefit from you in his life more often."
