Snails die most of the time from fluctuations in salinity/specific gravity and poor acclimation techniques. The hermit crabs are definately not killing the snails if you don't see any hermits actually living in the snail shells.
Low specific gravity is also a killer of snails. Are you using the correct formula to find out your specific gravity? Temperature affects it. I know my hydrometer always read 1.024 and after I do the math, my temp of 82 degrees bumps up the specific gravity to 1.026.
Snails have a difficult righting themselves if they fall upside-down, and are easy prey for other hungry inverts. However, they should rarely fall upside-down on their own, if ever. If you see them on their backs frequently, it may be because they are dying.
The mexican turbo snails are very hardy and excellent cleaners. If you ever need to replace your lesser snails, go with all mexican turbos (about 6 in a 65 gallon tank) and you won't regret it.
ps- your water readings seem fine, as long as they are stable. With the exception of ph, all of them must remain constant for good snail health. pH tends to lower at night
