I haven't done the opening ADR at Chef Mickey's, but have at Cape May. We were able to check in just a few minutes before our ADR time and then we all stood around the entrance and our servers came and got us to show us to our tables.
I wouldn't get there too early or you will just be waiting around. I would think 6:45 would be find. There aren't going to be an entire restaurant of people checking in at 7 because the ADRs are staggered so that each server doesn't get sat with an entire section at the same moment.