How to Make It
Heavily coat a 9-inch pie plate with cooking spray; set aside. Place crumbled cookies, salt, and 2 tablespoons of the granulated sugar in a food processor, and process until finely ground, 10 to 15 seconds. Add melted butter; process until mixture is moist and clumps together easily, about 5 seconds. Press crumb mixture evenly on bottom and up sides of greased pie plate. Refrigerate until set, about 30 minutes.
Cut part of the watermelon into cubes to equal 3 cups (about 16 oz.). Cover remaining watermelon with plastic wrap, and refrigerate until ready to use. Place cubed watermelon in a blender, and process until smooth, about 20 seconds. Pour through a fine wire-mesh strainer into a large measuring cup, pressing gently to squeeze out juice; discard solids. (You should have about 1 1/2 cups of watermelon juice.)
Place tapioca in a spice grinder or coffee grinder; process until finely ground, about 40 seconds. Sift ground tapioca through a fine wire-mesh strainer into a small bowl. Set aside 2 tablespoons plus 1 teaspoon of the tapioca powder. (Reserve remaining tapioca powder for another use.)
Stir together watermelon juice, 2 tablespoons plus 1 teaspoon tapioca powder, and remaining 1/2 cup granulated sugar in a small saucepan; let stand 5 minutes. Bring to a boil over high, and cook, whisking often, until tapioca dissolves, 5 to 6 minutes. Immediately remove watermelon mixture from heat. Let stand, stirring occasionally, until thickened, about 20 minutes. Stir in lemon zest, lemon juice, and (if desired) 1 to 2 drops food coloring gel.
While watermelon mixture stands, beat 1 cup of the heavy cream and 1/4 cup of the mascarpone with an electric mixer fitted with whisk attachment on high speed until soft peaks form, about 50 seconds. Gradually add 1/2 cup of the powdered sugar, beating until stiff peaks form, about 10 seconds. Working in batches, gently fold watermelon mixture into whipped cream mixture until smooth and blended. Spoon watermelon mixture into prepared pie plate. Refrigerate until firm, about 2 hours.
Scoop out flesh of remaining watermelon with a sharp 1-inch melon baller, making about 10 (1-inch) balls. Cut watermelon balls in half, and place, cut side down, on a plate lined with paper towels; set aside.
Gently stir together remaining mascarpone cheese and 1/3 cup powdered sugar in a large bowl just until combined. Beat remaining 1 cup heavy cream in a medium bowl on medium speed until stiff peaks form. Gently fold whipped cream into mascarpone mixture.
Dollop mascarpone-whipped cream topping on top of pie, leaving a 1-inch border. Place watermelon-ball halves, cut side down, along edges of whipped cream topping. Serve immediately.