also, as a practical thing, there really would be more controllers in the tower and at least one of them would be awake.
Forgive me if I left too much room for interpretation when I said, "if the same thing were to happen at 5 o'clock on a Thursday afternoon". What I meant was not that the lone controller had fallen asleep, (because that was not known until later), but if at 5 p.m. on Thursday, for reasons unknown, the tower was unresponsive to any and all attempts to communicate with it.
And again, we're addressing the specific question of whether the PICs' excercise of their authority in this case might be determined to be contrary to the applicable regulations, not whether safety was compromised. I suspect you'd agree that there's no ironclad relationship between the two.
Perhaps that will change your answer, perhaps not. If you are 100% confident in your knowledge of how the FARs apply to the specific situation, it shouldn't. Certainly, whichever way this all plays out, it will have been an educational experience for everybody...