Eating the roast and doorkey

I rewatched this today, it's one of my favourite films. I've just about worked out all the plot twists but one thing puzzled me. When GP says she will go out for a burger and MD says their maid has made one of her famous roasts and he thought they'd eat before the game. Then he asks her to wish him luck. If he's then going straight out how are they going to eat the roast?!

Oh, and curious that the killer had the same front door key as GP's. He had quite a low rent apartment and theirs was luxurious. Unless all US front door keys are the same?