Boil shrimp with seasonings, let cool then peel and devein. Cut shrimp into small pieces, or shread. Set aside.
Weigh squash, determine microwave time at 6 minutes per pound. Place squash, whole, on a glass pie plate. Microwave on high for 2 minutes; pierce rind in 4-6 places with an ice pick. Turning squash over, halfway through cooking, microwave on high for the time determined. Let stand while preparing the rest of the recipe.
Place mushrooms in a 2-cup glass measure; cover with vented plastic wrap. Microwave on high 1 3/4 minutes. Remove mushrooms and add 1 tablespoon margarine to liquid. Blend in flour with a wire whisk, then add milk. Whisking midway through cooking, microwave on high for 2 minutes, or until thickened. Stir in sherry, cheese and pepper. Add cooked mushrooms and shrimp, mix well.
Cut squash in half, remove seeds. Use a fork to scrape strands of squash from the rind. Place strand in a 1 1/2 qt casserole, pour mushroom sauce over squash; toss to combine. Place remaining 1 tablespoon margarine in a custard cup; microwave on high 30 seconds, or until melted. Blend in corn flake crumbs. Distribute over the top of squash. Microwave on high for 1 minute, or until hot.