Thank you Master++,

Your solution was precisely right!

Furthermore, I was indeed forgetting "that it is, after all, a validatecommand", which was my downfall.

Your answer also illuminates the reason that the printf was causing a successful outcome, namely that the return of printf, like that of print, is "true if successful".  Hence, a successful printf was the equivalent of returning 1, without doing so intentionally.

Thanks for helping me to get past that obstacle!