There are few things I think need to be stated:
1. C. irritans does not have a magic 'self destruct' switch at X concentration of copper, or X specific gravity. The copper level recommended by Seachem when dosing cupramine is considered the best balance between an effective level that is still safe for you fish. They recommend maintaining 0.5 mg/L, but that does not mean at 0.49 mg/L all the parasites are going to be hunky dory. Copper, like any medication, is about trying to raise the levels high enough long enough to kill all the parasites without killing the animal you're treating, and there will always be statistical variability within a range for that.
2. Without a copper test, you should not - I repeat - should NOT be dosing a copper product. At best, the levels will be too low and you're stressing your animals out for no reason. At worst, you'll overdose and kill them.
3. Of all the ways of treating parasites, copper is without question the most stressful for the fish because Copper is a potent metabolic poison to marine animals. The tank transfer method is probably the least, or second least stressful (chloroquine might be the best, but I have no idea how toxic it is to fish). What's more stressful on a human body - running outside naked in the winter from your house to your neighbours house once every three days, or consuming low doses or arsenic for three weeks? Fish have spent millions of years evolving efficient fight/flight mechanisms that quickly ramp up adrenaline, then mop it up when the danger has passed. They have not spent millions of years figuring out how to metabolically cope with 'therapeutic' levels of copper. Whether you can see it or not, your fish are experiencing organ damage to some degree, which they will not experience during the tank transfer method. Copper, done right (i.e. with regular testing of copper levels) is a perfectly valid treatment and can be situationally appropriate it's really hard on your fish.
4. If you're fighting ich, you need to be isolating your QT tank from you display completely. Stop doing water changes with display tank water. That's like trying to bail out a boat by displacing the water with sand.
5. You might be seeing more spots now because the protozoans they were infested with in the display are simply maturing. You might also not be maintaining high enough levels of copper and introduced more infective theronts from the display during a water change. Copper is only effective on one or two stages of ich's life cycle, once it's burrowed in to the fish, there's not much that can touch it.
