Not a GSB student but one thing I can tell you is what I heard at most schools about majors...dont worry about them. Take the classes you want, the ones that interest you and you think will be helpful. In your essays including a major isnt a bad idea if you want MC then strategy is usually a good one to have since many people go into strategy consulting.
GSB live, I went last fall I will say it definitely has its good points and weak points. Good, it overall is a good time, you meet lots of applicants and they go somewhat beyond the presentation you will get when they travel to different cities. They did a mock application walk through to kind of show you how they do it which was rather interesting BUT they didnt tell us whether they would have let the person in or not which was annoying. The bad, there was no class or anything it was just all applicants...if you want to visit a class set up a day near the event with a student or else you wont get to see a class. Also the interactions with students I would say is lower than a typical visit since during a normal visit the group of applicants present is MUCH less. Honestly, I think a visit on another day would probably be just as, if not more helpful than the live event but thats just my opinion.