Wait, when you say that "about 7 remain" do you mean that the snails actually died and you saw this and removed the shells, or that you just don't see them anymore. If "naso" snails are nassarius, then its possible that they are just hanging out under the sand: often they will only come up in response to food when they are actually hungry, or won't come up until later on after feeding. They also tend to be more active at night.
And this one small little crab: are you saying that it's a hermit crab, or some other unknown species? If you are finding truly dead snails, a crab could well be a possible suspect.