You can leave after your interview - just need to check out with admissions before you go. No final presentations, so all just depends when you are called. You won't know who's interviewing you (physician, PhD, or student) until you are called. I believe they also asked at the beginning of the day if anyone specifically needed to leave early for a flight, so just let admissions know and it sounds like they should be able to accommodate you.
You can take the GUTS shuttle to the Metro. There's a Metro stop for Reagan off the yellow line. That way you won't have to deal with crazy DC traffic and rush hour. But I'm not sure how long the Metro would take and I know you California kids just love public transportation!
I attended the optional lecture since they take you after the optional breakfast, which pretty much everyone went to. It was fine - our lecture was on platelets. Since I graduated last year, it was nice to know I could sit in on a lecture without completely going stir-crazy. Also an opportunity for you to scope out a good chunk of the class. Good luck!
I did - just sent them through the admissions office addressed with my interviewers' names.