Regarding addressing emails, I usually try to follow the cues of how they're signed by the professor, as others here have said. What I find useful (and had a British-educated, prim and proper professor suggest when I was contacting people under her supervision) is to start off by addressing them as "Prof. X" and if/when they sign an email with their first name reply with "Dear Bob (if I may),". I've had a few go back to signing emails "Prof. X" indicating that I may not, but I don't think I've ever caused offence.
Then again a lot of these people are Canadian and at smaller institutions, so your results may vary.