Copper did not kill your fish. I have had many flames in copper, and coral beauties. I had two die that shook their heads like yours did. And conclusion was possibly cyanide poisoning. Because the one I saw in the last moment went around is strange circles, and odd swimming before dying in an instant after that ordeal. The second one I didn’t see how it passed. But both had the same head shake your fish had.
Having read through many of the posts.
What are you using to measure copper? Just the Salifert? What are you using to measure ammonia? API? How are you dosing prime? As in device. Just the cap or measuring it by the bottle instructions?
I have QT’ed countless amounts of angles of all types, not a single issue with copper. Have QT’ed as much as 18-20 fish in one tank at once (I will not recommend this! Only reason was a ich outbreak that started infecting all the fish big time). Angles, tangs, clowns, wrasses. Never had copper issues. And have even done prime when I could not get a WC that day. No deaths. Now I never just dosed at free will. I always use measuring devices that I made sure are accurate in reading by using a precision calibrated graduated cylinder to verify my syringes.
For the past two years I have not even done ramp up for copper, I use copper power, and dose to 2.3-2.4ppm and put fish in directly. Currently have two Blue damsels, three dwarfs angles (flame, CB, and a golden angle) , and a powder blue, and a black cap basslet, in a 20 long. All are eating, no stress, and living life in medicated water. All were ich infected from the tank they were in prior.
My theory is prime masks ammonia, ammonia never leaves the system because it literally has no way to leave it, and then spikes back with a vengeance, once the prime is not working any longer. Had a skimmer been present it might have helped. But most QT systems are not big enough or have a possibility of a skimmer to be added. Either way I have seen prime when I would dose it, again emergency use only, only hold back ammonia for about 24 hours no more. I’m not a chemist to be able to tell you in detail what takes place 100%, but it would seem prime binds it temporary then after about 24 (was the case once for me), it comes back because it’s not being taken out of the system. So I never would wait up the 24 hour mark. I did what I had to get a good 50% water change for the tank before the 24 hour period.
My new method is now seeding Polyplab Genesis rock in a healthy disease free tank. And boy does the bacteria work off these rocks in QT tanks at maintaining ammonia levels. Other super nice thing they don’t hold on to chemicals or meds. So when dosing copper, no absorption. Dosing prazi nothing is going to stay in the system, same with antibiotics if needed. But you get the benifit of large amounts of bacteria population. I now even have loads of pods in my ongoing QT thanks! It’s nuts! Have not seen anything like this. The rocks really do work.
. 
