UPDATE (so to speak):
UPDATE (so to speak): Desperation drove me to try the Live TV update, despite concerns that we might end up with even less program data than before. It worked. For future reference by others who might be facing a similar issue, here’s what we did:
First I re-ran TV Setup, went through the entire selection reconfirming our previous settings. I ignored the CableCARD steps, figuring (hoping, really — a second concern was that we might have to start jumping through the CableCARD pairing hoops all over again) that it had already been paired long ago and didn’t need re-pairing. After getting the channel lineup information, I let WMC download the guide data, fingers crossed.
This time it worked! Despite a worrisome message that the guide could not be downloaded, upon clicking on Finish I went to the guide and we had 13 days of programming for all channels. This is actually about three more days’ worth of data than we’ve had in the last several months.
Then things got “interesting” again. I honed in on a particular current program and selected it to watch. Instead of getting a show, we got a “tuner conflict” error message claiming that all four tuners were in use. 
Huh? This was impossible — we were neither watching anything at all, nor recording anything. What’s more, the Ceton Diagnostic tool informed us that only two of the four tuners were actually tuned to any channel. It was also having problems with the CableCARD “entitlement,” the last step in that part of the diagnostics.
After pondering our next move, we decided to shut down the PC completely (not “restart”) and reboot. Launched WMC and got the usual “no tuner” message which we have learned to deal with by unplugging the CableCARD, waiting 20 seconds, plugging back in, and switching to another channel.
And now we have all our cable and local channels back, plus almost two weeks of program data. 
WAF has shot way back up… for the time being.
Michael, thanks for the suggestion.