As an old friend used to say, “Hecky darn!” I had really hoped that would work. The Option+Command+R boot should have eliminated any boot drive issues — at least in theory.

The only remaining idea I have is a Genius Bar appointment at the nearest Apple Store. They will have an external drive they can boot your Mac from which should unquestionably take any “you cannot erase the boot disk” question out of the picture.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein