1. I have never cleaned the rear element of any of my lenses. Mainly because they are not designed to be cleaned by the hands of the average consumer, and if you make sure to seal it when not in use, why would you clean it?
2. As said before, send it back. That is not normal for ANY lens, especially of L quality. Keeping that on your camera body, or even continuing to shoot with a loose element like that, could damage the inside of your camera thus costing you more than the lens itself.