Bring juice from canned pineapple to boil. Whisk together eggs, yolk, and sugar in medium bowl. Add to pineapple juice, whisking continuously over medium heat until it becomes thick (3-4 minutes). Take off heat, stir in butter and salt. Pour in small bowl, cover with plastic wrap and refrigerate for 1 hour.
Bake cake according to package directions. When cooled, slice horizontal into 3 even layers.
Take vanilla frosting, orange juice and orange zest; stir to combine.
Place bottom layer of cake on platter, spread 1/2 of orange frosting on bottom layer, top with mandarin oranges.
Place another layer of cake on top, spread pineapple curd over this layer and top with 4 slices of pineapple rings cut into 1/2.
Add third layer of cake, frost all around the sides with rest of orange frosting. Frost top with cream cheese frosting.
Layer 4 pineapple rings on top, cut into 1/4 and second can of mandarin oranges.
Press toasted coconut into sides and on top of cake.