Hmm. After a couple of reboots (I forgot to mention above) that didn't change anything, I prepared an external HD to start up from a clone when I noticed Airport being back up again. Rebooted from the clone anyway, and all was well there too, so no software issue. I wonder if the MBP's Airport card needs to be reseated. Something to check if the issue recurs. Meanwhile, my wife's happy again (she hated the ethernet tether, or perhaps the only chair within its reach).
Meanwhile, while we're on the topic, I still like to know the answer to the question I posed above.