Prepare and bake cake according to package directions, using an ungreased 10-inch tube pan. Cut cooled cake into 1 1/2-inch cubes. Set aside.
In a bowl, sprinkle gelatin over cold water; let stand for 1 minute. Add boiling water; stir until gelatin is dissolved. Add the orange juice concentrate, sugar substitute, lemon juice, lemon peel, and salt; mix well. Refrigerate for 30 minutes or until partially set. Fold in 2 cartons of whipped topping.
Line a 5 1/2-quart bowl with two overlapping pieces of plastic wrap, letting plastic wrap hang over edge of bowl. In another large bowl, gently combine the cake cubes and whipped topping mixture. Spoon into prepared bowl, gently pushing against side to prevent holes. Cover and refrigerate for at least 24 hours.
Just before serving, uncover bombe. Invert onto a cake plate. Remove bowl and plastic wrap. Frost with remaining carton of whipped topping; sprinkle with coconut. Garnish with mint, cherries, and lemon and orange slices if desired.