I'll be curious to know how the chiropractor went. That last video, where he's free lunging wiht an English saddle on (which is too far forward, btw), he still looks really stiff. He moves both back legs almost in unison, and hardly brings them under his body at all. I don't think that's normal for a QH, is it? I mean, I know they are soemtimes not big movers, but that looks like his pelvis is really locked up.
Will he step under himself if you you disengage his hindquarters? Watch how he turns around when on his own, if it's really stiff, or can he step under his own body.
Does he lay down and roll? And have you seen him actually roll all the way over?