Semi-formal. For an explanation see this website, which is the first hit from the Googles. (But it makes sense). https://www.thespruce.com/semi-formal-attire-guidelines-1216504
Yes. And there is a tent for inclement weather so unless it's raining sideways, be prepared to be outdoors for a bit. It's September in New England, so it could be 40 or 90 degrees. (If you're not from New England, definitely bring a wrap or a jacket because as Ellen has learned, you're never safe up here without a jacket. Even in the dead of summer).
Yes. We highly recommend getting a hotel in the North Shore area. If you do stay in Boston/areas South, you may hit pretty bad traffic on the way in.