I never understood why people thought Magus has to die for Frog to become human again. I've watched the scene many times and never got the impression Magus died - just that he disappeared and went to do his own thing without the help of Crono's team. Besides, there's reasonable evidence in Chrono Cross to suggest Magus is still around. I would guess that Magus, after journeying with Crono/Frog/etc., found Frog to be a nice guy after all and removed the curse voluntarily. Just the way I see things.