I must say,it's hard to think there isn't a danger of the capstone sliding off when you see this picture from 11 years ago http://www.themodernantiquarian.com/site/11/trethevy_quoit.html#images
(the last one in the last line but one, posted by phil).
I can only presume the EH engineers have calculated that friction will keep it in place.
So far as the other movement is concerned, have you been able to replicate any of the early pictures on TMA in order to show a change?