If looking down is easier, then why does Google Glass not put the display there? We've had bifocals for a while now, with the reading glass part below and the distance (or plain, if not short-sighted) part above and these work pretty well.
I've used google glass quite a bit. Even developed for it. I never experienced the eye pain suggested by this article except when first trying it. However after developing and testing on it all day the problem I experienced was when I tried to go to sleep. When I shut my eyes I could still see a white rectangle of light burned into my eye by the google glass screen. It subdued after a day but was quite annoying. I can't imagine that having a bright screen that close to your eye is good for you.