I've never had high Mg levels bother snails, even when I took it to 6x the Ca level. I agree with those who say your tank is too clean for them to survive at this point, and with those who pointed out that they are the wrong species of snail for tropical temperatures.
Give your tank time to mature before adding snails back to the display. A few small hermit crabs will probably survive fine in a clean tank if you're feeding enough.
When you're ready for snails in the display (i.e., you have some natural alga growing on the glass and rockwork), find some trochus snails. They are long-lived and very hardy. A couple Nassarius snails can also be good for helping to keep the sand bed loose and clean. Just be sure there is food for them.