Make the custard by combining pineapple juice, sugar, eggs, flour, and ½ teaspoon salt in a saucepan. Cook over medium heat until thickened, stirring constantly.
Remove from heat and stir in lemon juice. Cool completely to room temperature (about 1 hour).
Bring a large pot of water to boil with remaining salt and oil. Cook pasta until al dente (5-7 minutes), drain and rinse with cold water.
In a large bowl, combine pasta, custard, drained oranges, and all pineapple. Gently fold in whipped topping.
Cover and refrigerate for at least 8 hours or overnight.
Just before serving, fold in marshmallows and coconut.