oops, my post should have said shrimp are more delicate, but I see you understood me anyway.
so yeah, if you smell dead things then I would say that you are correct. something has died. dead snails would cause water problems and that may have affected your shrimp between changes.
As for your snails I know people sell them on ebay so that may be an option. to make sure you don't get anymore from the pet store as the person to clean them to make sure that you don't bring snails home with you. It's no guarantee but it will lessen the amount you may bring home.