Sounds like a plan I would make. And have done so in the past. Do you have an idea where you'd have your breakfast/brunch in GC? Only place I know that opens at 7 am for breakfast is Sunset Cafe at Grand Case Beach Club or Rainbow. I'd opt for Sunset Cafe, quieter, beautiful view and setting. Beach there is clean and crystal clear. They have a watersport rental kiosk there so you could make a morning of it. Not enough time for lunch in Anse Marcel in my opinion. Plus nothing else there except the beach.

Around 1 pm I would head to Orient Village and beach it there with drinks at any of the places along the beach. Pick one of the restaurants in the Village that opens straight thru lunch to dinner, eat then head back to the ship. Traffic is heavy going from P'Burg to Orient but not as heavy going back to ship. Since you have to board by 5, don't push it incase there's a traffic problem.