Well, healthy snails mean it's not Copper, that should be a relief.
The Xenia will uptake Iodine but it would take a massive OD to harm Xenia, it probably appreciates the OD (if that's what it is). The dead Hermit syndrome is consistant with NM's suggestion of Iodine overdose.
Last I knew Combisan didn't have much of anything in it so that's probably not a source of Iodine. Perhaps a bad batch of salt? I'm just guessing here. Perhaps a food like Nori or squid?
I would suggest you spot feed your shrimp some high quality food for a while. Molting that often is going to take a toll on their health after a while.