I feel you. My tanks fallow right now too. Its been 5 weeks, I have another 5 weeks to go. After this long of a fallow period every fish is going through a QT. Much like you, corals are not my problem it's the **** fish!
All those white spots sounds like when I had velvet, specially the time line of death. A freshwater dip, followed by a Ruby Reef dip gave them temporary relief before going into QT. It allowed me to still bring copper levels up over a few days without them passing.
I set up my QT for "cheap". Amazon has the biowheel 350 for about 30 bucks, 40g breeder for $40 during Petco's dollar per gallon sale and a heater was all under $100 bucks. Keep a new biowheel filter in your sump at all times and you will be ready to go when need be.