OK, I went back and messed with the region mute tool. You don't use it above the timeline, and you don't use it on the
bottom of an individual track (or lane), and don't expect it to work on multiple tracks (or lanes), either. It works if you swipe it on the top half of a track (or lane) and if somehow you've accidentally muted a stack of lanes, you get to unmute them with this tool one lane at a time.
So I did this, and I could see the waveforms normally and hear the takes. Then I tried to comp the vocal again. This time all was well,
until it wasn't, which happened at a different place near the end of the timeline, at which point I was hearing all the takes at once. (Also, randomly during comping the backing tracks were muted for a few bars, although luckily not erased.)
I deleted the end of the vocal and sang it again, punching in in overwrite mode where necessary. Maybe I've had too much coffee. Something in SONAR is screwed up, and this forum has always been a great place to get help, thank you all, but with the developers all gone it just seems pointless to spend a lot of time figuring this out. I've got what I need right now, so I guess I'll count my blessings and mix it down before something else goes south.