How to contact an Emeritus professor?
Chances are, if they are not answering their university mail address, they are either busy or can't be bothered. They are retired, so both is completetly ok (it is sort of the point of retirement that you do not have an obligation to react to work mail anymore). Trying to get in touch with them over different media is probably fruitless, and somewhat obnoxious. Besides, I would really hope that the university administration does not give out private addresses to the request of random people.

Edit: an almost completely unrelated side notice: emeritus profs can be insanely busy in some cases. There is a famous austrian physics emeritus who is now writing popular science books, breeding alpacas (!), touring Austria with a science-based stand-up comedy show (!!), producing and starring a TV programme, and waging an ever-lasting war against the catholic church (as head of Austria's atheist movement).