I don't think you are going to open the back and fix a white spot on the screen by yourself, unless you replace the screen assembly.
New assemblies aren't too expensive, but there isn't much sense in doing that since it's under warranty.
It seems to me that you have two options - go to the MS store, or send it back in.
I am not sure what else you really want out of this thread at this point. For some reason, going to an MS store doesn't appeal to you, so that leaves you with opening a warranty case and sending it back.
I don't know if you will get MS to send you a new phone in advance, but I sort of doubt it. You can check the 830 threads - some folks there had success with warranty exchanges. In some cases, folks walked up to the MS store (though they did not buy at the MS store), and they got an exchange for their phones on the spot.