We never had any trouble getting a bus to make early ADRs. I don't know exactly how early they start, but we had several ADRs a full hour before opening and always made it with plenty of time to spare on the busses.
At MK, there is a separate entry area for people with early ADRs, they will start letting those people in a little before 8am. There are large signs showing you where it is, and the CMs will probably be announcing it as you walk up. Everything you read says to bring your confirmation # - we have never been asked for it, but I would have it just in case. They usually just usher everyone in who says they have an ADR.
Be sure to take some pics going down Main St. and by the Castle, it will be the only time you can get pics without the crowds!