Line the bottom and sides of a 9-by-9-inch baking pan with aluminum foil, allowing 2 to 3 inches to extend over the sides. Lightly spray the foil with the cooking spray.
Melt 8 tablespoons of the margarine in a large saucepan over low heat. Stir in 4 1/2 cups of the marshmallows and cook until almost melted. Add the pinch of salt. Remove from the heat and cool for 1 minute. Stir in 20 of the caramel slices just until they begin to melt. Stir the rice cereal into the marshmallow mixture until the cereal is thoroughly coated. Press the mixture evenly into the prepared pan. Set aside to cool.
Melt the remaining 3 tablespoons of margarine in a small saucepan over low heat. Add the remaining 3 cups of marshmallows and cook, stirring often, until everything has melted and the mixture is smooth. Spread the mixture over the cooled treats using a lightly-greased spatula. Cool for 20 minutes or until the topping is set.
Lift the treats from the pan using the foil sides as handles. Place them on a cutting board and sprinkle with the sugar if using. Cut the treats into 25 squares with a lightly-greased large chef's knife. Top each square with one of the remaining caramel slices and sprinkle with the sea salt.