In a medium-size saucepan, stir together the sugar and cornstarch. Gradually stir in the half-and-half until well blended. Heat over medium heat, stirring, until the mixture thickens and comes to boiling; boil for 1 minute, stirring constantly. Remove the saucepan from the heat. Stir in the crushed pineapple along with its juice and the rum. Spoon into a bowl. Let stand until cooled to room temperature, stirring occasionally, for about 45 minutes.
In large bowl, beat together the heavy cream and sugar with an electric mixer on medium-high speed until stiff peaks form. In a 14- to 16-cup glass trifle bowl or clear glass dessert bowl, layer one-third of the cake cubes, followed by one-third of the pineapple custard. Sprinkle with one-third of the reserved mandarin orange juice. Then layer on one-third of the orange pieces, one-third of the whipped cream and 1/2 cup of the toasted coconut. Repeat layering 2 more times: cake cubes, custard, orange juice, orange pieces, whipped cream and coconut.
For a garnish, sprinkle the remaining toasted coconut and the reserved orange pieces over the top of the trifle. Cover and refrigerate until the trifle is thoroughly chilled, about 2 hours. Garnish with fresh mint leaves, if desired. Makes 16 servings.