Bring water to a boil over medium-high heat in a saucepan. Stir in 1 cup chopped fresh or thawed frozen mango. Remove from heat, and stir in 1 Tbsp. fresh lemon juice. Process mixture in a blender until smooth.
Preheat oven to 325°. Whisk together sweetened condensed milk, lemon zest, 3/4 cup mango puree, 1/4 cup lemon juice, and mango schnapps in a bowl.
Beat egg yolks with a handheld mixer in a medium bowl at high speed 4 to 5 minutes or until yolks become pale and ribbons form on surface of mixture when beater is lifted. Gradually whisk in sweetened condensed milk mixture, and whisk until thoroughly combined; whisk in buttermilk. Pour mixture into prepared crust.
Bake at 325° for 20 to 25 minutes or until set around edges. (Pie will be slightly jiggly.) Cool on a wire rack 1 hour. Cover pie with lightly greased (with cooking spray) plastic wrap, and freeze 4 to 6 hours.
Top with Sweetened Whipped Cream and chopped fresh mango just before serving.