Cara, you have hit on some possible solutions on your own. If you can contact the original owners/breeders of her and get them to shed some light on her background while she was with them before they sold her, it would be of great insight into her behavior. Also have the vet check her over, just to be on the safe side and rule out any physical impairments.
You have decided to give her one more chance, for which I commend you. Try the hackamore on her and see if that helps with some of the reining problems.
When I was growing up, one of my friends had a purebred Arabian Mare that everyone said was a total fiend. She had some of the worst ground manners and was almost impossible to ride once saddled. He gave her to me, and I studied this mare for awhile, then I slowly started to work with her, retraining her.
Yes I had my share of bruises from her, but in the end she turned out to be one of the best horses I ever had. Part of the problem was that my friend had no idea how to treat her, one day he was all nice with her, and the next when she did something he didn't like he roughed her up. So she mistrusted people. I had to win her trust, and once she realized that I wasn't going to hurt her, she started to come around. Once I gained her trust, she would do anything for me, she was absolutely fearless. I know alot of people would rather have a gelding or a stud horse, but me I'll take a mare anyday, moody or not.