Oh boy. Sorry for your loss. I'm going to give you a few basics and then give you a link to a page that will answer all of your questions.
Basically, Ich is in just about everyones reef tank that doesn't use a quarantine/hospital tank prior to putting new fish in it. Fish are often able to fight off an ich infection easily provided they have stable parameters and are not stressed. I do believe that the temperature swings and/or the crab may have contributed to your problem.
Ich has a lifecycle where it attacks the weak or stressed fish, gathers enough food to reproduce and then falls off the fish. Yeah, the Ich is gone!!! Unfortunately, no it isn't. It is just in another form. It will be back with a vengeance and if you have fish in the tank that are still stressed, they will get attacked.
If you are willing to take that crab back to the LFS and fix your temperature swings, then you can likely solve this for the long term. PS---I don't think you need a chiller, just a heater with a thermostat.
I'm sorry to say this but a CBS is not a cleaner shrimp. It won't clean anything off of your fish.
Here's the article, I hope it's helpful. Again, I'm sorry for your loss.
http://www.wetwebmedia.com/ichart2mar.htm 
