So the additional info on the genetics doesn't help narrow it down unfortunately, it's simply proving that all those things ARE a possibility, that's why hidden genes are so interesting!
Don't feel guilty! You're doing SO much for this mare and I can completely understand what a frustrating situation this must be!! She is SO lucky to have you and this foal too! I would ask him flat out at what price he would be willing to sell her for. I would remind him that she used to be yours and you've done x y and z to help out at no charge to him. I wouldn't fight with him just say you regret selling her and want her back. A price similar to what she would sold for would be more then fair given her condition (it SHOULD be much less but if he wants her so badly...). Pretty much any horse can produce blue roan (black + roan), the special thing about your mares color is she guarantees a cream gene (so ironically this foal CANNOT be black at best it can be smokey black, unfortunately that can look the same). So there's nothing special about her color that another horse can't produce if that's what his goal is.
I am not surprised the only vet doesn't know him! Yikes! I'm glad you have a good vet who is taking this seriously and will check the guy out, hopefully there's something that can be done and if the vet himself makes a report it will carry a LOT more weight. I'd give it a little more time and then touch bases with him. I sort of hope the other animals are in similar condition, just because that way something can be done about it. At best he is providing minimal care but I hope it can be proven he is providing less then minimal so someone can step in.