It sounds like you've covered a lot of bases, but I think there are still alot of holes in your basic training here. You are giving him an awful lot of excuses that would suit a human for disrepectful behavior. I would probably fall back to groundwork, like someone else mentioned. If someone brought me this horse for training, I definitely wouldn't jump on his back till I found out what was making him tick and eventually explode. There are many little things jumping out here, from his behavior in the round pen to how he reacts to being tied up and his inability to adapt. Sure, physical issues could contribute to his intensified defensive behaviors. I don't think its a matter of "teaching him a lesson". Horses tell you exactly whats wrong before you get on their back. You are interpreting everything that he's doing in your language. It says something completely different if you look at it in his.
Just to cover my bases, lol he does cross tie perfectly. He just doesn't single tie very well, as he is very fidgety, and smart enough to let himself loose, lol. I'd imagine that he was never single tied on the track, he was always either held or cross tied.
I totally agree--he's basically telling me that he's never done this (i.e. Regular riding with a person sitting on his back) before, and he doesn't like it anymore. I know that he knows how to walk, trot, canter, and gallop and transition to those gaits in a relatively smooth fashion, he knows how to lead, yield his hindquarters, and forequarters, back, turn left and right, etc. etc.....the problem is not the groundwork, as he does these things perfectly when asked...its getting him to re-associate these things with his new life, not his old one on the track. That's his problem and essentially why he explodes, because that's exactly what he was trained to do, so he thinks its right. Like I said in the OP, I need to change his thinking, not just change his actions.
Ironically, its his actions that are right, but his motivation is wrong. For the last six months, instead of wiping his slate clean, which is what I need to do, I've been trying to build his new skills on top of the old ones....basically like putting an American plug into a European socket. You can jam it in there and it might work, but in this case you're probably just going to get electrocuted.