Allow egg whites to stand at room temperature for 30 minutes to create more volume. Meanwhile, preheat the oven to 250°F (120°C) and line a baking sheet with parchment paper. Draw a 9-inch circle on the paper and invert it so the circle is on the reverse side.
In a stand mixer fitted with the whisk attachment, beat egg whites, cream of tartar, and salt on medium speed until soft peaks form. Gradually add 1 1/2 cups sugar, 1 tablespoon at a time, beating on high speed until stiff peaks form and the meringue is no longer gritty, scraping the bowl as needed. Beat in the lemon juice and rose water, then gently fold in cornstarch.
Spread the meringue over the circle on the parchment paper, building up edges slightly to form a shell. Bake for 1 1/2 hours without opening the door. Turn off the oven and let the meringue dry in the oven with the door closed for another hour. Remove and cool completely on the baking sheet placed on a wire rack.
In a large bowl, toss sliced mango and strawberries with 2 tablespoons of sugar. Let stand for 20 minutes.
In another large mixing bowl, beat whipping cream and mascarpone cheese on medium speed until soft peaks form.
Place the cooled meringue shell on a large platter, spread the cream mixture inside the meringue shell, and spoon the fruit mixture on top. Garnish with edible pink rose petals and serve immediately.