Preheat oven to 350°F.
Combine graham crumbs, sugar, and melted butter thoroughly in a large bowl. Press the mixture firmly into a 9 inch pie pan, coating the bottom and sides. A heavy juice glass will hlep do this. Bake on middle rack until the crumbs mixture has darkened a bit and the ingredients have fused to form a crust, about 10-15 minutes. Remove from oven and let cool completely on a rack. Reduce oven temperature to 325°F.
In a large mixing bowl beat the egg yolk until they are lighter in color, about 1 minute. Add the condensed milk and 2 tbsp cream and stir to incorporate. Add the sour orange juice, sour orange zest, and 4 teaspoons navel orange zest. Stir until filling is creamy and light yellow in color, about 1 minute.
Pour filling in crust and bake on middle rack until surface looks like jello, about 14-16 minutes. Transfer to a rack and cool for 30 minutes. Transfer to a refrigerator to cool completely, about 4 hours. Cover in plastic wrap if you plan on keeping pie longer, eating within 24 hours is recommended.
Before serving, whip the remaining 2 cups cream with the confectioner's sugar. Spread over pie and sprinkle with remaining 1 1/2 tsp navel orange zest. Slice into portions and top each with a mint leaf if desired.