I believe you can add more LORs as long as you still have spots left within your 4 possibilities. You can delete anyone as long as they haven't started anything, but once they have you can't delete it.
Also, I called and was told apps (and additional materials) are mailed out every Thursday. GPAs verification have been a lot faster. I know someone who submitted their app on the 8/25 and was verified on 8/29.