I'm not sure that you have anything major to worry about. Your nitrates are not the issue.
Astraeas can be weak and die from shipment and poor acclimation to your tank. Beyond that, if you don't check your tank at least twice a day and turn over the upsidedown ones, the hermits will take advantage and eat them. Some hermits may attack and eat them just to see if they'd like to try their shells.
Overall, Astraeas are a bad choice for a reef tank because of the fact that they can't turn themselves over. A better algae eating snail is a Trochus, Strombus Grazer, or Collonistas. Try to avoid buying Astraeas in the future.
Hermits usually are pretty hardy when acclimated properly. Given that you mixed species of hermits, you might have had them fighting and killing each other. I doubt there is a new disease in the tank. In a 55, you don't need more than a few hermits. If you have bristle worms, and scavenger snails (like nassarius), you really don't need them. For example, I have 3 hermits in a 180 tank and 1 in a 80 tank.
As you know, whenever you collect your own critters and plop them into your tank, you run the risk of introducing problems and diseases. Whenever I have introduced empty shells, I boil them in water for several minutes.
Best of luck,
Roy