Darn, too bad about the Smartpak thing! My horse thinks they're practically treats (ha).
You could try looking at some of their ingredients (in SmartCalm, etc) and seeing if you can DIY supplements, although honestly I don't have any recommendations on what to mix it with-- you've tried so much! If sweet isn't working... have you tried maybe a mineral salt + some kind of bran + supplements? Just guessing here!
I have heard fantastic things about Pentosan. It's great because it not only helps to restore joint lubrication, but it's also an anti-inflammatory agent. I'd also recommend pairing it with a dietary hyaluronic acid (or HA injections)-- but be careful. Some of the lower molecular weight dry HA supplements are no good (they're broken down before they're absorbed), so look for a liquid or gel (I use hyaluronex).
For the exercise issue... when I use my chiro, she recommends NO tight circles and NO lunging afterwards, so I'd say try to stick to straight lines or see if you can free lunge him in an arena (where he won't be going in small-ish circles). In my experience, for a horse with chiro problems = circles are bad.
Good luck with your boy! He's quite handsome!