I take it "tact" is not a word that's in your vocabulary,
@Colin_T ?
However. . .
I do think we have to face the possibility that Colin's right.
Leaving the previous Betta in the tank overnight and not draining the water and disinfecting the tank could certainly be the cause of some of the problems you're seeing now, certainly.
Eating a rock, given the fact he hasn't are anything else, is almost encouraging. Almost. I wonder if it's got caught in the gut and caused severe constipation? He might not then want to eat.
I wonder how hard a fish could bang its head, unless he swam right into it? If it was aimless, I'd have thought the water would certainly have slowed down his descent.