. . . Yep, the kitchen ceiling leak was from the newly-unclogged washbasin. Maybe when the obstruction was dislodged it took part of the pipe with it. Or the clog was protecting a weak place in the pipe.
However it was, yesterday morning I only used enough basin water to wash my hands and brush my teeth, and the crack in the kitchen ceiling was dripping again.
Same plumber is coming back tomorrow afternoon. Fortunately, it counts as a followup call and won't cost me another service fee.
And God willing he can get at the bad pipe through the back of the bedroom closet. The home warranty company will redo plaster or drywall, but they won't pay to replace demolished wall tile. And I am not ready to redo my john!