Ah, I think I have a better picture of what all's going down for you guys and how it's like on a day-to-day basis. I appreciate the time you spent to write those posts, my questions were answered generously and I think that helps us both.
"My hubby works in the oilfields of North Dakota."
Whew ... rough gig. Especially in the Winter.
I feel quite a bit more certain by now that the important problems remaining in your family are the ones between your husband and his ex. It sounds like all of the other problems have either been satisfactorily mended, or are on their way towards mending and looking hopeful.
In addition to the deep distrust that has grown in your husband towards his ex, there is also some mysterious force at work between the two of them that causes them to push each other's buttons. Something about the nuances of their personalities that just doesn't mesh well. That's one thing that would be worth digging into and gaining a closer look at. Who knows, there might be little things they could both do to reduce the button-pushing factor.
You mentioned that the three of you have been able to sit down together a few times and talk about things. That's a good sign. If it could be done, I would actually try to do a sit-down each time your husband is home for a week. Like, establish it as a formal habit. A standing agreement between the three of you. That would mean your husband and his ex would have to agree to it, and you can't decide for them that they'll agree. But I'd suggest the idea to them, and encourage it when you can. Each and every sit-down is an opportunity to practice communicating, as well as investigating the sources of friction and perhaps discovering new insights about the joint relationship.
Of course you don't want to do a sit-down during one of her blow-ups, you want to look for an occasion where everyone is calm and things are going relatively well. If you're willing and able to be a mediator for the other two that's fine; let's work on getting them communicating fruitfully on that level first, and then maybe eventually they can practice talking to each other directly a little at a time. Certain rules should be in place: obvious things like no interrupting, take turns so everyone has a fair chance to talk, no raised voices, no name-calling, etc. ... and some things that are more subtle, such as not trying to assign blame (to self or others), taking ownership for one's own feelings, making efforts to put oneself in the other's shoes, repeating back what the other person said so they know you understand and are listening, etc.
All these rules come naturally when it's just you and the ex talking together, or just you and husband talking together. It's when all three of you sit down together that such rules must be especially kept in mind and formally agreed to.
Not that everyone's going to abide by the rules perfectly all the time by any means; there will probably be many slip-and-falls, especially in the earlier conversations you guys have. So it's important to be patient and persistent in picking yourselves up and dusting yourselves off. If things get too heated, maybe take a break, but come back together and try again.
Keep a list of the rules the three of you feel you should follow (including perhaps a time frame for the sit-down, e.g. an hour might be all you can emotionally stand at first), and perhaps make three copies of the list so you're all looking at it while you talk. In addition, each of you could prepare for the "monthly sit-down" by writing your own personal list of wants, needs, and/or concerns you want to talk about during the next session.
Sometime when it's just you and your husband talking, when things are going pretty well, I'd consider sitting down with him and having a heart-to-heart about where his intentions and commitments lie. Is he putting up with the ex just to hang on to the kids? Is he willing to try to mend things with her if she's willing to make improvements? If not for her or his own sake, would he be willing to try for your sake, and at your request? You might share with him (in a non-accusing way) the vision you have for your triad, and how much it would mean to you if he could help you attain that vision. I suppose a similar conversation might be a good idea between you and her too, sometime when you have a good chance.
Another thing to discuss just you and your husband, is the disabilities of his ex. Yes, disabilities, and not just of the chemical kind either. There is something broken inside her mind, something that makes her lose her grip with rationality. I'm sure he'll agree with that, but the challenge for him is to try to imagine what it must be like to be her, to be angry and stressed out much (most?) of the time, to be pushed to the very edge of her failing ability to hold it together and stay rational and calm. I doubt these things make her feel happy inside, and I have a feeling she doesn't choose them; she doesn't think, "I'm going to be a crazy insane bitch today, and make the man of my life as miserable as possible."
It is my theory that it's a vicious cycle when she can see herself losing control, and can't do a thing about it; it just makes her madder and crazier and more stressed out than ever. If she really reaches down, she probably feels ashamed of her outbursts, and wishes she could make them stop happening. Instead, she just goes on hurting the man that she loves, and that's got to leave her feeling guilty, miserable, defensive, and unhappy. The point is, she has a tougher life to live than it may seem to the uninitiated or on the outside. She is not as strong as your husband is. She wouldn't last five minutes out in the North Dakota cold. But she is probably trying as hard as she can to be a better person (when she's not too discouraged or freaked out to try).
She can probably easily imagine (and I'd prompt her to do so, in private between just you and her) how it would feel to be her ex. Working harsh conditions for a month at a time away from home, missing his family, only to finally have a week off and instead of kind, appreciative treatment from his ex, he has to dread more of her outbursts, more of her searing anger towards him, of her never forgiving his sins (or even misunderstandings) of the past, of, in a word, her turning into a porcupine in his presence so that he can't even get near her. She needs to try to imagine how much she hurts his feelings. Always pointing out his faults, never his good features. Maybe on some level he misses the happiness (and there must have been some, long ago) he once had with her, and would like that back, but just feels like it's impossible and could never happen.
The point is, encourage each of them to try, as hard as it is, to put themselves in each other's shoes. If they can even do that a little, it might help prepare them to work with you as a mediator and have those monthly sit-downs. (Weekly sit-downs, if your husband's future schedule ever allows for it!)