First of all Cat, I'm sorry you're having to go through this. You did mention concerns about this couple that when they met with you they had an "It will all work out", attitude, but not much in the way of details to back it. At least you're getting a prime example of how they cope with troubles and problem solve....not very well obviously!
My ex-couple are not married, but have lived together for over a year. They were originally in a V relationship...with her as the hinge between her husband and our guy. She subsequently divorced her husband when he no longer wanted a poly relationship. My couple consider each other to be "husband" and "wife" within their poly relationships. I moved partially to be closer to them and to work further on our relationship. (His and my relationship...and to build a deeper friendship with his other partner.) I lived with them for the first month here and then found my own place...which was our plan right up front. Once I moved out I saw less and less of him. I raised this issue with him numerous times and he'd repeat that he didn't want me to be in a secondary role, but to just give him some time because he was getting adjusted to a new job. (as was I!)
Well come this past December, 4 months after I moved, he told me he wouldn't even commit to seeing me once a week as I had asked for, because life was just too hectic and busy and he didn't see an end to that anytime soon. He just couldn't stretch himself that far. I had offered to do the driving to his area so he wouldn't have to spend the hour on the road to where I live. I offered for some of "our" time together to be family time, including his partner, her son, and/or her mother, etc. (We'd done this on numerous other occasions. I'd even babysat for her son on one occasion when both of the parents had to work on a Saturday.) I offered to be flexible as to what day of the week it was that we'd get together on a week to week basis. (I'm no longer married and my sons are adults who live on their own, so I have much more time and flexibility. Her son is 9 years old and she was just finishing up her nursing degree and holding a job.) It was still a "no go" on his behalf. I was sick at heart that he apparently cared so little for me or was so self-centered. He didn't see either as being true!
I had discussed my move with both of them at different times throughout the process of interviewing for a job in the area and before I accepted the job offer in the area and then moved. I wanted to make sure I wasn't pushing myself on them and that BOTH were okay with my being closer and more involved with the family. They had invited me to stay in their home the first month until I could search around and find a place to rent....I didn't ask them. I made sure I was out of their house at the end of the month's time. He'd told me several times he didn't consider of me to be a "secondary" and didn't want me to feel that way, even prior to my moving closer.
Sooooooo...'nuff about me and my relationship with an established poly-couple!
With regards to your guy
.....perhaps you can just ASK him what he'd like from you or if there is anything he can think of that would be helpful on your behalf. Put it back in his hands. Don't try and mind read because, if he's feeling powerless right now in the other relationship, he may be taking it out on you and you'll never quite get it "right". If he can't come up with anything, just tell him if he does, to please let you know and in the mean time you'll just leave the issue alone unless he says something to you about wanting your help/input, etc.