I only have a few things I can say about this:
1: You need a round pen and you need to do some Parelli/Roberts/Anderson type stuff with her. She needs to learn you are the herd leader and you will protect her. That will giver her confidence. When she has confidence all your problems will be solved.
2: I have a friend who took a Percheron x ? Cross from a PMU situation. She had her 4 years. She was NEVER able to do much with her even after 4 years. She did all of the Parelli type work, everything she could think of. She had experts come in to work with her, she tried everything. She was never able to vaccinate her, treat wounds, have her feet trimed, nothing. When they had to treat her for something once, they attempted to tranquilize her. They gave her an amount that would have KILLED most horses, and she never even got sleepy. The mare was impossible, and dangerous. She had originally wanted to use her as a broodmare as she threw tanks for babies and the babies were very kind. The mare seemed to like the company of humans. She would come up and watch people when they were working, she would let you stroke her shoulder with one finger, but on her terms and if she decided that was unacceptable, a person could get hurt. My friend decided that she would give her to a a home that could basically just turn her out and let her live out her days as she was so unhandleable. But no one could get her on a trailer. Trust me when I tell you this woman tried everything humanly possible, this horse was just not going to cooperate with anything. After 4 months trying to get her to load in a trailer, my friend decided it would be unfair to pass this horse's problems on to another person and she made the very difficult decision to put the mare down. I know, that sounds really harsh, but it was for the better for the mare. She could not be treated for illnesses, she could not have her feet trimmed, she would have ended up living her days miserably with people feeling bad that they couldn't help her. Not to mention all of the horses she could have injured or gotten sick from her not having her vaccinations and such. In this case it was the right thing to do. I am not in ANY WAY suggesting you put your mare down. I just find it interesting that you are having similar issues with the exact breed and history my friend had problems with. I think there may be something mentally wrong with these horses that goes beyond a lack of training or a severe fear. I question the equine version of schizophrenia (sp?) in cases like this. Horses that don't come around after everything that was put into my friend's mare, likely never will.
If you have not tried the natural training approach, I would take that route and give her at least 6 months. If you have... then I am sorry and I don't know what else you can do.
It saddens me that these horses come out of these situations like they do. People do all they can, but some simply can not be saved.
I hope I did not scare you too badly, I just want you to know that sometimes, they don't come around.