The shell theorem

Hi,

I found this derivation at wiki: http://ift.tt/1exd5EP



Can someone tell me if the theorem is valid only for a solid sphere , or

the pull of a hollow sphere is exacly the same it would be if all the masse where at the center wrt to a mass at any distance D from the centre?

does it apply also when D = R?

If it does can you explain how (apart from the math derivation) if ,say, an electron is on the circumference it gets the maximum pull, and it suffices that it moves 1 mm toward the centre, that pull vanishes all of a sudden? How can such minimal difference in distance cause such a huge result?

Thanks