What would eat what's in the sand and would hermits and snails eat what's on the sand
Like Webster1234 said....the two main critters that eat the life in the sand are the sand sifting Starfish and Gobies. The Conch, Goatfish, Sea cucumbers, and some Snail and Crab species can deplete the life too. If you set one up...do some research before you add anything. DSB's are pretty cool to watch and if you can resist the urge to stir the sand, then it might be an option for you. My primary reason for setting one up was for denitrification. I soon realized that Siporax could accomplish the same goal. As long as you control your feeding and skim the tank, you really don't need to setup a DSB, but they absolutely work.
The infauna and CUC in a successful DSB is very diverse though. 200+ larger species are commonly found in a mature DSB. Ron Shimek tested a DSB he had in a 45 gallon tank and found 90,000 to 150,000 smaller critters that were about 1/2mm+ or about one fiftieth of an inch. I'd bet there's a lot more than that.
If you do decide to go this route PM me and I can give you some advice when you get started.