It seems like there are as many ways to cure Ich as there are fish that have it. Yes stressing fish out by moving to a QT tank just to cure it and then stress it out by moving back seems counter productive. The reason the fish got Ich in the first place is becase it was stressed/sick in some way. The goal of the QT tank is to get rid of the Ich and nurse the fish back to full health. If the fish is healthy the move back to the DT will not be enough stress to cause the Ich to come back. The reason people see Ich on multiple fish is from a systemic tank problem causing all the fish to stress out.
Ich can enter the tank from muliple routes, new rock, new fish, inverts, etc. Keeping your tank healthy is the first step, keeping the fish healthy is the second. This isn't to say you wont see it but a healthy fish will be able to slough it off a lot quicker than a sick one.
I had a recent bout with Ich and all I did was raise the temp a bit, varied the food I was feeding to include food with garlic, did more frequent water changes and reduced the amount of time the lights were on. None of this seems drastic, because it isn't.
You can do the hyposalinity thing as long as you are not running a reef tank and as long as you have no LR
or sensative inverts, the LR
will die off and cause other problems in the tank. You can try copper or some other chemical treatment but again this depends on what you have in the tank. I would rather not put any chemicals in my tank if I can avoid it. Ultimately the choice is yours when it comes to fixing the problem. You just need to decide on the cure and act on it before all your fish get it and die.