Sorry to hear about this
Hope this gets better, that snail thing has me thinking that when I set my tank back up there will be a limited clean up crew.
Wow, sorry to hear, Dawg this sucks. I don't think it was the kalk treatment, though, I nail aiptasia in my old 20g long nano/quarantine tank regularly and have never had a problem. The ammonia could have spiked right after the snail died and caused trouble then, and then been nitrified by the live rock. By the time you tested the ammonia it would have been long gone. The high nitrates make sense in that case.
Sorry to hear, but in such a small tank, there was prolly nothing you could have done to save them anyways. It could have happened that fast, so even if you had been on top of it, and done a waterchange it's possible the damage was already done.