I bet they would understand without it, but a very brief adendum probably wouldn't hurt. As I understand it that's sort of the rule of thumb with these kinds of addendums. Keep it brief and it can't really hurt. It might rven help a little.
I'm not sure. I would recommend checking Top Law Schools Harvard Law applicant forum from either this year or last. They likely have a list of frequently asked questions.
I do know that the interview is a very good sign for your chances.