Skyhuntress, I really highly recommend against oral supplements. They really do very little for the amount of money they cost, even in clinical trials. I would recommend monthly IV/IM injections of Adequan and Legend, or just the Adequan. These injections kept Bully sound with ringbone, and are doing the same thing for my mum's current horse who was on oral supplements and on and off sore with his owner (he is now 100% sound). I have my horse on them for a preventative and Anebel was on them when we had her too. We get them for a little under $125/month injected by a vet per horse (we don't get the name brand though). When you look at these oral supplements, that's easily what you're going to be paying anyways for something that doesn't work nearly as well.
If you are going to just go the oral supplement route, the only thing that has really had enough clinical testing to be called "proven" is Corta-FLX or Corta-RX. Even then they don't work nearly as well as the injections, they still work. I would grab the more "high-powered" one (I totally forget which one that is), even hough it's more expensive, it's worth it.
Good luck!
Also - for your question about the supplement removal, all these supplements do (in theory) is stimulate joint fluid creation and thickening, and improve the cartilage. When you stop using or injecting the supplements, the body's function was not affected, just assisted by the supplements, so gradually they will leave the horse's system and it will go back to "normal". This means that the joints will begin to deteriorate normally and inflame normally as well, so you can end up with a lame horse if you keep the workload high when you take them off supplements. All they basically aim to do is slow down or stop joint deterioration, so when we remove them, everything just naturally ages.
But, with the steroidal therapies, these can actually deteriorate the joint by only reducing the inflammation in the joint without actually strengthening anything, so the horse will over use it (in theory) and cause damage.
And if you have any questions about what's legal, I think I'm pretty up to date on that. Just stay far away from the "organic/natural" stuff because that will mostly all test. The Adequan/Lengend combo is fine, along with the Corta-FLX and -RX.