In large bowl, combine shrimp, crabmeat and sherry; mix gently to coat. Cover; refrigerate 1 hour to marinate.
Heat oven to 350°F. Spray 13x9-inch (3-quart) glass baking dish with nonstick cooking spray. Cook rice in water as directed on package.
Meanwhile, melt 2 tablespoons of the margarine in large saucepan over medium heat. Add shallot and mushrooms; cook and stir 5 minutes or until tender.
Reduce heat to medium-low. Add remaining 6 tablespoons margarine; stir until melted. Lightly spoon flour to measuring cup; level off. Add flour, seasoned salt and pepper to mushroom mixture; mix well. Cook until bubbly. Gradually add half-and-half and milk, stirring constantly until mixture boils and thickens. Remove from heat.
Add seafood mixture and cooked rice to mushroom sauce; stir gently to mix. Pour into sprayed baking dish. In small bowl, combine topping ingredients; mix well. Sprinkle over top of casserole.
Bake at 350°F. for 30 to 35 minutes or until casserole is bubbly and crumb topping is golden brown. Let stand 5 minutes before serving.