It didn't happen in 24 hours. It's been happening and getting worse since you got the fish. It's had ICH the entire time. The ICH parasite goes through different life stages. free swimming stage, egg stages, etc.. The ICH is free swimming and attaches to your fish and actively starts feeding on the fish. That is when you see spots on the fish. after a few days the parasites begin to detach and drop off the fish (the spots go away) and then the ICH lay eggs (cysts) which attach to the substrate. Copper doesn't kill ICH cysts/eggs. Then after a few more days the cysts in the substrate begin to hatch and then the ICH swim up and attach to your fish again and you see spots again, probably more than last time.... then they feed for a while on the fish again before they drop off to lay more eggs, rinse, repeat... the ICH just keeps multiplying and attacking the fish worse. The spots come and go, but the ICH was still there and getting worse. Just because you didn't see spots one day didn't mean there was no ICH. The ICH was probably just in a egg stage not the free swimming/feeding on your fish stage. So going from no spots to spots and a dead fish, didn't just happen in 24 hours, it's been happening since you brought the fish home with ICH from the LFS.
That's why you need to run copper at the correct therapeutic level for a full 30 days. The copper doesn't kill the cysts/eggs, only the free swimmers, so you have to keep the fish in high enough level of copper long enough to kill the free swimming ICH, and then allow enough time for all the cysts to hatch and become free swimming ICH, which the copper can then kill. If you don't keep the fish in copper long enough, more cysts will hatch and reinfect the fish. You have to keep copper going long enough to outlast all the ICH and ICH eggs in the tank. Generally 30 days is advised. You might be able to do it in a shorter time, but 30 days is best practice.
If you dose the QT with the proper amount of copper, you will begin to kill the ICH in the QT tank. First it will kill the free swimming ICH and then when the eggs hatch, the copper will kill them too. so you can start that process now, even before you get the new fish.
So I would go ahead and dose the QT tank with copper to the correct level now. Then when you get a new RG, put it in the QT tank and keep it there for 30 days. The 30 day clock doesn't start till you add the RG. by the end of 30 days, all the ICH should be gone, provided you NEVER let the copper level dip below the therapeutic level... if if does at any point, then you have to raise it back up and start the 30 day timer over. When doing any water changes, you have to dose your new water with copper to the proper level before adding it to the QT tank.
Or option #2 would be to drain the tank, bleach it, and start over but then you'd have to cycle the tank again and this would require a lot more new saltwater.