The Pac-Man Guide in the FAQs section has a useful tip - labeled, literally, as "Useful Tip" - near the beginning for how to "corral" Pac-Man with magnus he can't eat, and thereby control exactly what he eats. --- - Vilurum Xaren, leader of the happy turtles, and knowitall. ... And if you can't spell "Vilurum", call me "Vil". :)
I always thought that it was very funny that you had to "imprison" him so he doesn't eat your stuff. Even thought you are warned before you pick him up.
About the missables this was taken from that same guide: "The magnus Warm Cheers, Icy Jeers, and Tub-Time Greythorn are missable after a certain point. Feed these to pac-man as soon as you can, or you can never complete the quest."
--- "I call upon thee in the land of the dead, to unleash thy fury of thunder.... Indignation!!!" - Genis Sage