A) Cover your screen with a thin layer of metal. Set your desktop wallpaper to plain black. Turn your screen brightness to maximum. Turn on your room lights. In areas where the screen is black the reflected light will dominate, acting as a mirror. In areas where the screen is illuminated the transmitted light will dominate, acting like a highly glossy screen.
B) Write a bit of software to set the webcam image as your desktop (properly reversed). The operating system might not be able to handle a full thirty frames per second, but even one frame per second would be neat.
C) He didn't say that he tried to scan a mirror. He just said that scanning a mirror wouldn't work. Maybe he didn't want a thousand answers from people with less physics knowledge suggesting to try just that.