Holy crap! Ok so I assume you haven't changed feeding schedule. Feed any fresh veggies lately and forget to rinse? Cold water could cause stress but it couldn't account for the original dead ones. Were the plecos harassing the shrimp at all? (doubt this as well but thought I would check)
You mention java moss, is this your moss? If not, is it a new purchase. If so, did you have any issues before adding the moss?
Before the 70% WC when was your last WC? Any parasites, planaria, etc? If they were larger shrimp could have been old age, but I doubt a # that high. I don't know what to say I am speachless...you lost all your RCS and your crays are showing symptoms?
Your water company make any changes recently?? Something important you should know about...
As for salt I guess it can't hurt, might damage your plants. Salt may help keep them from getting any infections.