The ich parasite need fish in order to survive. If only some of the fish in a tank show signs of ich, ich is present in the tank. If you only treat some of the fish in a QT, the ich is still present in the main tank and will stay alive by feeding off the other fish that don't appear to have a problem. That is why the only real way to break the whole life cycle of the bugger is to remove all fish from the tank, treat all the fish, and let the tank go fishless. That way you kill all the ich on the fish, and all the ich in the tank.
Quarantining fish for a month will not guarantee a fish does not have ich, even if it shows no outward signs of it. But normally, within a month, if you have a diseased fish you'll see it. The stress from the new environment, new food, new everything... is usually enough to bring everything out in the open. It's really no different than us - if I get super stressed out, I'm pretty sure I'll be getting sick soon!
Also, you mentioned "heat treating" the tank. I'm thinking you're referring to the "heat the tank up to speed up the life cycle of the parasite" treatment. All that does, supposedly, is speed up the life cycle (metabolism) of the parasite - just as it does everything else in the tank. It doesn't kill it.