Sounds like he had ich or something. And the Maracyn is probably what killed your crab. Your new puffer is suffering because you have some kinda parasite or whatnot infesting the tank. He is now infected. You better get him out of the tank and into a QT. Id post pictures of him on this site and try to figure out WHAT he has before treating with more stuff. ALot of those meds are HARSH.
If this fish dies too, id wait a few weeks before putting any more fish in. You may need to completely redo your tank since you put Maracyn into it.
Sounds like terrible advice from a vet to just dump Maracyn into the tank like that. It needs to be measured and done in a QT tank, no? Poor crabs.
Id just bury the fish and say a prayer. Id deffinately not try to cremate a fish and then wear the ashes. That just sounds weird to me.