Saute the mushrooms in the 2 tablespoons margarine in a large skillet over medium-low heat for 5 minutes; remove mushrooms from skillet and set aside.
Melt the 1/4 cup margarine in the skillet; whisk in the flour.
Stir in the half and half, green onion, mace, parsley, salt and hot sauce; cook until thickened, stirring constantly.
Stir a small amount of the thickened flour mixture into the egg yolks; return mixture to the skillet.
Cook a few minutes longer, stirring constantly.
Adjust seasoning if needed.
Remove from heat.
Fold in the mayonnaise, mushrooms, crab meat and shrimp.
Spoon into a buttered 2 1/2 quart casserole.
Bake at 350 degrees for about 20 minutes or until hot and bubbly.