Very interesting! All are excellent, just 2 things: 1.'The man who I met..' is a little unnatural because it's a short, simple sentence, so without 'who' is better. 2. … is a teacher' if he's still relevant to you - you're going to see him again or something, … was a teacher' if it's just a past story and he's gone from your life. (one more thing, many 'academics' believe "whom" should be used with objects, and because of that people write it in newspapers, say it on the radio, etc - but they're wrong. It's really unnatural and no one says that in normal life - only if they're told by someone that it's "more correct". Really, "whom" is only used before a preposition, like 'to whom..')