A quick tip...
I know the thread starter has said it's going back but I thought I would pass this little hint on anyway.
I thought my new iMac had dead/stuck pixels when I first booted. Well, they weren't. The guilty "pixel" was actually dust. To verify this, make a note of where the pixel is and turn off your screen. In light conditions, take a look at where the guilty pixel was located, and if you can see a speck of something between the glass panel and the screen, it is likely not a dead/stuck pixel.
Removal of the glass panel can be done quite easily and with a can of compressed air, the speck can be blown away.
I would challenge that almost every new iMac made suffers from this "problem". I've seen this on EVERY iMac I have looked at and that is quite a few! Even if you are lucky and there is no dust under the panel now, it is likely that some will eventually appear in the future since the panel is not sealed, rather it is held in place by magnets. Fortunately it can be cleaned with care, but you do run the risk of smashing the glass panel.
Just a point I thought was worth making...