Thank you everyone, for your thoughts.
~ I don't have a helper, but someone made a video of their horse by hanging their iPhone around their neck... I will try to get some video of him moving.
~ same vet as one of them (my main vet) but a different chiropractor. I don't think the vet took me seriously the first two visits, as I tend to cry wolf...a lot. When I really got into talking with her about my concerns, she seemed to take it seriously, so here's hoping. CC is (gulp) at the ready.
~ This is what I mean by camped out... the first pic was taken about a week after he arrived, the second from a few months ago. To be honest, I had to search for pics of it. Mostly he was standing under himself, but I think he stands like this quite often, more often than seems normal.: IMG_0740.jpg DSC_1220.jpg
~ Will do on the EPM check.
~ . What they have done so far is ask me to walk and trot him about a bit, then, since nothing is apparent and he is definitely not "lame", they pronounce him okay. The chiropractor was quite interesting; there were some crystals involved at which I inwardly roll my eyes, but she also did some neck and back work which provoked an interesting response from Boojum; when she was done with each stretch he would repeat the exact movement by himself one or two times, as if testing the new situation or sensation. I asked if there were any issues I should be aware of and if he was okay to ride, and she said he was in fine shape, no issues.
~ Yes, he was tested for Lyme about a week after he arrived here: he had a nasty tick on his neck. That was also the first 'vet check' for odd movement. I never considered eyesight, but a problem there would help explain some of his behaviors for sure. So I will keep that in mind when they come out also.
The back shrinking has been gone for at least a year. I think it may have been the way I was grooming him. I got more aggressive brushes and stopped being so gentle, and he now likes being brushed. And he is cleaner :)
But one big thing I neglected to mention in my original post, and the thing that convinced me it wasn't just clumsiness or me being over-cautions was that on the lung-line (we have started mini-lunging: three minutes a side at a walk, then two minutes a side at walk & maybe trot) is this: he understeps with his right hind. His left hind tracks right into his left front hoofprint but his right hind lands behind his right front hoofprint.
This happens both directions. Some days it is only by an inch or so, but some days it can be by as much as 9 or 12 inches.
Our space is limited so our circles are small (hence the slow speeds). One time it looked as though his entire right hind leg kind of pivoted outward a bit, and he took a really lame step, so we quit for the day.
If I can figure out how to take and post a video of him moving, I will do that.
Thanks again for the ideas on how to go forward with this.