Devein shrimp, if desired.
Bring 6 cups water and boil-in-bag mix to a boil in a Dutch oven; add shrimp, and cook 3 minutes or just until shrimp turn pink. Drain and rinse with cold water.
Cut a 1-inch-wide strip from the top of each baked potato. Carefully scoop out pulp, leaving shells intact. Mash pulp.
Melt butter in a large skillet over medium heat; add green onions and next 3 ingredients, and sauté 5 minutes. Add half-and-half; cook, stirring often, 2 minutes. Stir in mashed pulp, shrimp, 1/2 cup Cheddar cheese, salt, and pepper. Spoon into shells, and place on a baking sheet. Sprinkle evenly with remaining 1/4 cup Cheddar cheese, breadcrumbs, and Parmesan cheese.
Bake at 350° for 20 minutes.