I'd say no. Disliking a major isn't really a good reason for an addendum, imo. Just apply and see what happens. if they for some reason ask you about it in an interview then just tell them, but yeah don't do the addendum
Wait, you took a proctored exam in June, received a 170, and are still one point below the median for northwestern? I'm missing something... either their median went up, you don't mean official exam by "proctored" exam or something else...