In a measuring cup, combine the reserved mandarin orange juice with enough water to reach the 2-cup line and bring it to a boil. Remove from heat and stir in the orange sugar-free Jell-O until completely dissolved.

Transfer the gelatin mixture to the refrigerator and allow it to chill for about 20-30 minutes, until partially set but still pourable.

Pour the whipping cream into a mixing bowl and add powdered sugar if not using Cool Whip. Whip on medium-high speed until stiff peaks form, about 3-5 minutes.

Once the gelatin is partially set, break it up slightly with a fork. Gently fold in half of the whipped cream along with the majority of the mandarin oranges.

Spoon the mousse mixture into dessert dishes or glasses and chill in the refrigerator for at least 30 minutes until fully set.