Well, I got the belt changed. The entire insides of the printer looked as if it were covered by dead ants!
Much to my surprise, when I turned the media lever bracket to the position it was when I removed the side panel and the mysterious shaft that came out, I was able to thread the shaft through all of its brackets with no problem. It seems also that the only function it serves is to rotate with the media-release lever. Its irregular profile in turn activates those brackets, which lift the roller assemblies to free the paper, and then to lower them again. My guess (and hope) is that I have not disturbed the alignment or function of the pizza wheels and pinch rollers. I'll find out soon enough, though.
One other question: It never occurred for me to check on how close to metal-to-metal contact the belt tensioner was before I loosened and removed its screw, so I don't know how tight to make it. Right now it's about 3/16 from the side panel, and the belt seems more than tight enough. I'll probably tighten it down a bit more to account for stretching of the belt in case it has a "break-in" period, but it seems to do the job of moving the carriage easily (by hand, at this point, of course.)
A further question: On tightening the T-10 screw recessed behind the circuit board in order to limit travel of the carriage assembly, I did not find any increase of resistance. I became worried that It might just keep turning until it popped out the other end (I've done lots of things like that in my clumsy history), so I just turned it enough to stop the carriage, and then added a couple of turns. Should it be turned until it stops?
Thanks again, and this morning I'm more hopeful that the printer will work.