Might be a reason this horse was abandonded...other than the owner was a worm, scumbag and a thief.
The OP seems to be doing all she can to get this horse evaluated. If it's pain hopefully that will come to light. If it's behavior maybe a trainer will be able to help. At 12 years old who knows what this horse has gone through.
Even if she doesn't keep this horse it is good to have some knowledge as to what may be the issue so as to not just pass on a big problem...like what got passed on to her.
OP, I wish you the best. And the best for the horse. Otherwise it makes for a pretty miserable life for him, and a danger for anyone trying to ride him.
If you ever find yourself in a fair fight, it's because your tactics suck. ~ Marine 1SGT J. Reifinger