This may seem like a silly question, but how does one address a Lama in an email? Or would it not matter because Garchen Rinpoche is not going to be directly reading it anyway?
Just be respectful, and keep to the point. I'm sure "Dear Lama" or "Dear Rinpoche" would be fine. You're right--he may well be unable to reply personally (either because of language, or because he is famous/busy), but perhaps one of his students would respond. Is there a FAQ file on his website somewhere...?