I've never used PolypLab's ich treatment - but I've also never heard of a treatment for ich that is both safe for invertebrates / corals _and_ effective against ich.
If you do find that you need to get some copper, I've had better results from chelated copper (CopperSafe), rather than the ionic variety (Cupramine). While treating with it, you'll want to remove the liverock, as it can absorb the copper and make your levels fairly unstable. Levels of copper in the water should be tested regularly, and with chelated copper the API test seems to be the one that works best.
You're spot-on about ich not being the result of stress. It's a living creature, and has to be introduced into the tank in order for our fish to "catch" it. Problem is, it's in a lot (most? all?) of wholesale / retail systems, so introducing it into our home systems is pretty easy to do.
~Bruce