I don't think you will be allowed in early with a reservation after the official park opening time.
When we went last April, we had a 9:30 reservation at Akershus and arrived for rope drop (I believe we got to the gates around 8:30) at the main entrance. We went to Anna & Elsa right at rope drop (but we walked at a normal pace, since we had older folks and walking young kids in our party) and were behind maybe 4 or 5 other families. We were done with the meet and in the check-in line for breakfast at 9:20. We did this on a morning where we were planning to do just the World Showcase, so we weren't missing out on any valuable touring time by eating breakfast while the park was open. We finished a leisurely breakfast, rode FEA with a FP, and then were getting off the ride a few minutes before the WS opened at 11.
Most of the people at RD seem to be headed to FEA, TT or Soarin' so if you headed directly to the A&E meet at rope drop, walking briskly, you would probably be done just in time for your breakfast if you can't find an earlier time. Akershus is just steps from the meet location.