Drain both cans of pineapple tidbits, reserving the juice.
In a small saucepan over medium heat, whisk the reserved pineapple juice, eggs, egg yolk, sugar, cornstarch, and vanilla until smooth. Cook, stirring constantly, about 10 minutes, until thickened into a smooth, glossy custard.
Transfer the custard to an extra-large bowl, cover, and chill 1 hour in the refrigerator or 30 minutes in the freezer until cooled to room temperature.
Cook the pasta according to package directions. Drain, rinse under cold water, and cool completely.
Gently fold the whipped topping into the chilled custard until smooth and fully incorporated. Add the cooled pasta, pineapple tidbits, mandarin oranges, mini marshmallows, and shredded coconut; stir gently until evenly combined.
Cover and refrigerate at least 4 hours, or overnight, to let the flavors meld.
Just before serving, top with maraschino cherries and extra marshmallows and coconut, if desired.