Acceptance of academic email endings
I don't have statistical data on this (and I doubt any exist). Personally I find "Best regards" and "regards" to be very formal, but it depends on who you're speaking to.

I prefer to use "Best wishes", "All the best", "Best" (if the email is brief or we're in a hurry) or "Ciao" (my supervisor is Italian).

As for salutations, I don't think Prof. Dr. is used anywhere outside of Germany, because usually only the most recent or senior title is used. In other words, use Dr if they're a PhD and Professor if they're a professor. If you don't know which they are, find out.