I had tried reinstalling Lightning a couple times and never got any response from the install - no dialog boxes popped up, no install status bar, no spinning wheel, nothing. Nor did the calendar or tasks tabs show up when I relaunched Thunderbird after trying to reinstall the add-on. So that was when I posted the above.
Went home, came in the next morning, launched Thunderbird and was immediately hit with 6-8 weeks worth of reminders.
I forgot to mention that I’m using Portable Thunderbird. The only thing I did differently overnight from what I had tried before my post was to remove/reinsert my thumbdrive and restart the Portable Apps Platform. I didn’t reboot the PC overnight, nor was there an auto-reboot. And of course I had removed/reinserted my thumbdrive and restarted the Portable Apps Platform many times prior. So it had to be some combination of the couple attempts to reinstall Lightning combined with removing/reinserting the thumbdrive, and relaunching the Portable Apps Platform and Thunderbird.
I figure the problem probably originated the last time Thunderbird auto-updated. It’s normal for it to tell me it can’t restore the add-ons and I have to do that myself - which is so trivial I don’t even remember what the steps are. But in this case, I don’t remember getting that advisory.
So apparently when I tried to reinstall Lightning, it must have tweaked something deep in the bowels of software Never Never Land and gave Lightning the nudge it needed to start working again.