if their teeth are overgrown snails wont help as they won't be able to bite them any way.. though you are right shelled foods should be fed at least once every few days to keep the teeth in check.. the only option is to cut them down mate.. you'll need to get some clove oil mate and something to cut em with maybe smaller clippers etc or I'm afraid these guys will starve and die. sorry but it will happen.
by the way dwarve puffers don't need any brackish water at any time.. they are fully freshwater fish.