Seating - ADR is an approximate time. You will be seated normally within 15 to 20 minutes. You can also check-in at the restaurant podium 15 minutes before your ADR time.
If it is a normal opening park day - 9 AM, get to the ticket turnstyles by 8:15 AM. Then Chef Mickeys ADR would be 7 AM.
May I suggest a 7:30 AM or 8:00 AM breakfast at Crystal Palace or Cindy's ?
An early breakfast in the park allows you to get into the park to get to your ADR before the general public. You can take pictures of an almost empty Main Street and the castle.
You will be done with your breakfast before 9 AM and will be able to get into which ever "land" on MK you wish to be in - when the clock strikes 9 AM.
We love getting into MK or EPCOT for an early breakfast, just for the above reason.