Combine first 3 ingredients in a small saucepan; stir well. Cover and cook over low heat 15 minutes or until plums are tender, stirring occasionally. Spoon plum mixture into a blender, and pulse 3 times. Cool to room temperature.
Preheat oven to 350°.
Place crackers in a food processor; process until crumbly. Add 2 tablespoons sugar, margarine, and egg white; pulse 5 times or just until moist. Press crumb mixture evenly into a 9-inch pie plate coated with cooking spray. Bake at 350° for 8 minutes; cool on a wire rack 15 minutes. Freeze piecrust 30 minutes.
Place an extra-large bowl in freezer. Remove ice cream from freezer, and let stand at room temperature while piecrust is cooling.
Spoon ice cream into chilled extra-large bowl. Fold plum mixture and sorbet into ice cream to create a marbled effect, and freeze 45 minutes or just until set but not solid.
Spoon ice-cream mixture into prepared crust; freeze until set. Cover with plastic wrap; freeze 6 hours or until firm.
Place pie in refrigerator 30 minutes before serving to soften. Serve pie with whipped topping.
