Well I guess we know who was the winner there. The bettamax is your best bet. I find Melafix to be to harsh for bettas and can only assume that it burns the labyrinth organ. Every time I have tried it the fish was dead withing minutes.
IME tank dividers are crap. We've tried them many times trying to keep guppies separated. Not once were they 100% effective. Someone always got to the other side. And like you I have no clue how. Could not see any gaps to allow this. So yeah, I would get another small tank for the other betta.