How would be the best way to do this? MK opens at 9am. Could we grab a bus at 7:30am to MK and then transfer to monorail to Polynesian? Would a cab be better?
Bus would work fine. That early, unless there's EMH, the buses will only be running every half hour - so yes, be waiting by 7:30. Ask the bus driver what the best way is to get to your Polynesian breakfast reservation.