Filling - Whisk zest and yolks in medium bowl until tinted light green, about 2 minutes. Beat in milk, then juice; set aside at room temperature to thicken.
Crust - Adjust oven rack to center position and heat oven to 325F. Mix crumbs and sugar in medium bowl. Add butter; stir with fork until well blended.
Pour crust mixture into 9 inch pie pan; press crumbs over bottom and up sides of pan to form even crust.
Bake crust until lightly browned and fragrant; about 15 min.
Pie - Pour lime filling into crust; bake until center is set, yet wiggly when jiggled, 15 to 17 min.
Put pie on wire rack; cool to room temperature. Refrigerate until well chilled, at least 3 hours. Can be covered with lightly oiled or oil sprayed plastic wrap laid directly on filling and refrigerated up to 1 day.
Optional coconut topping - sprinkle toasted coconut over pie.
Optional whipped cream topping - Up to 2 hours before serving, whip cream in medium bowl to very soft peaks. Adding confectioers' sugar 1 Tbsp at a time , continue whipping to just-stiff peaks. Spread evenly over pie with rubber spatula or decoratively pipe. Garnish with optional sugared lime slices and serve.