What a situation...
Well, with that update, honestly I think that you have just two options.
1. Downgrade your expectations of Rob drastically... to the point where you expect nothing of him so that it is a pleasant surprise when he does come through, rather than a crushing disappointment when he doesn't.
2. Make an effort to loop in Michelle. Because honestly it sounds like Rob is either not telling anything about the plans that you are trying to make with him, or he is not telling her the truth about your communications. (If he told her that you guys would be out of town and not available for a visit, that would explain why he was so angry/embarrassed when you ran into them at the restaurant!) Include her on all emails, calls, texts. Make sure that she knows about all of the things that you are trying to plan with Rob. Maybe she can give Rob a needed kick in the pants and get him to respond to your overtures, and maybe she will decide that it really isn't any of her problem to manage his relationship with you guys. But it is worth a try.
A couple of PPs have mentioned this, too. I'm seeing a lot of merit in this idea. Bopper also mentioned that Michelle may be the planner, and Rob isn't. I think there's a lot of truth to that, too. The pattern we're seeing is that they make plans to come to OurCity when Michelle has something planned with her family or if they want to attend OurUniversity's home football or basketball games. The sense I'm getting is that Rob will not commit to plans with us until he is absolutely certain that they are clear of anything that might conflict with Michelle's family. Given that, I do think we should loop her into communication about plans we're initiating. Essentially, we'd be working more directly with the planner rather than Rob as the middle man.
A couple of months ago, Michelle initiated a visit with us for DH's birthday. It was completely unexpected that she would contact DH herself. That hadn't happened before, and it surprised DH that she did it. It worked out fine. Everything was organized well beforehand, so there weren't any last-minute surprises.
A few PPs have mentioned that DH is behaving like a doormat. I would describe him as being very passive about some things, especially where the boys are concerned. One of his attitudes about the boys has been that he understands they have a lot of people they want to see when they're in town, and he doesn't want to pressure them to commit to plans. He wanted to be easygoing about their time, so he wouldn't self-advocate for time with them. I think the unintended consequence there is that he communicated behaviorally that it's okay for them to skip seeing him. On the other hand, he also has been terribly hurt when the boys throw him their crumbs of time. That did improve after the four of them talked a few years ago. The frustration I'm seeing now from DH where Rob is concerned is a relatively new development. What I think may help DH is to understand that he can't have it both ways with Rob: He can't lay down like a doormat and then be surprised (or upset) when Rob walks over him. We need to establish a different pattern if DH wants to see Rob more frequently.