but have you tried chemical behavior modification–I mean Regumate?
He certainly sounds scarey & dangerous. A friend of mine who runs a boarding stable encountered a horse somewhat like you describe–the difference being that this horse, although dangerous to work around on the ground, was a total dream to ride.
The horse was purchased at New Holland (shades of recent rescue/horse slaughter threads!) by a local riding stable & leased to a 13 year old girl. According to my friend this horse was a known biter so they kept a muzzle on him but he was a total dream to ride–this rather inexperienced 13 year old could ride this horse bareback cross country, he was that bombproof.
However, the day came when he managed to grab my friend by the jacket (he slammed his muzzled muzzle against her chest hard enough to grab her through the small hole in the muzzle) & she was TOTALLY CONVINCED he tried to kill her. He was put down.
Your story & your horse certainly fit in with these recent threads & I hope some of the rescue people will give you some input because you have raised an important question:
Can every horse be saved?