Excuse my conveturbation, but I have to circle back around to say that I actually got it working!
I was about to quit and let an RMA take care of it, when I discovered the same 125v coming out the neutral of my good, functional switch. Also, the neutral wire had come out of the wire nut bundle. Apparently, this is normal behavior for these switches. I put my new, not-yet-functional switch in the old place, and it worked fine, and I didn’t see the voltage out the neutral. (I also cancelled the RMA.)
So then I needed to figure out what I was doing wrong with the wiring of the three way. It had to have something to do with the neutral. The location of the switch shared a box with GFCI outlet. And the bundle of white wires in that box was not doing it for me. The outlet there was getting a neutral in from the bundle, and sending a neutral back out into the wall. I ran my neutral from the my switch to the second back hole on the neutral load out, and the switch started working! (Mea culpa: I do not know why. I’d welcome an explanation.)
A complicating factor was that every time the neutral would go live, the GFCI breaker would open. I hadn’t noticed that previously. That meant many of my tests weren’t telling me anything. And I needed to be careful with the order in which I restored power at the circuit breaker panel. The outlet and the switch, in the same box, are on different circuits.
Anyway, that’s my Horatio Alger story. Getting two pendants on a three-way switch definitely wasn’t worth the effort I put in to it, but after tacking on the sense of accomplishment that comes with figuring this puzzle out, well, then maybe worth it.