On the lack of foreshadowing for the "Riddle can't kill Riddle" curse, there was enough stuff around for me to generate a similar hypothesis last year (admittedly with prompting).

The great-grandparent comment did make me consider unbreakable vows as a theory of what happened on Halloween. E.g. to prevent one of his Horcruxes from later killing him, Voldemort made an unbreakable vow not to magically interact with his alter egos (this causing Harry's sense of Doom around Quirrell). Doesn't seem necessary, though.