Dissolve the orange Jell-O powder in boiling water, whisking for about 2 minutes until fully dissolved and no granules remain.
Add cold water, stir to combine, then refrigerate the mixture for 20 minutes until cool but not yet set.
Whisk in the dry instant vanilla pudding powder until smooth. Refrigerate for another 20 minutes until slightly thickened.
Fold in the Cool Whip in two or three additions using a rubber spatula, using slow wide strokes to keep the mixture light and airy.
Drain the mandarin oranges and crushed pineapple thoroughly. Pat dry with paper towels to remove excess liquid.
Fold in the drained mandarin oranges, pineapple if using, and mini marshmallows gently until evenly combined.
Cover and refrigerate for at least 1 hour, or overnight for the best texture. Stir gently before serving and garnish with extra mandarin segments if desired.
