Mix sugar and cornstarch in heavy large saucepan. Gradually add lemon juice, whisking until all cornstarch dissolves. Whisk in eggs and yolks. Add butter. Stir over medium heat until curd thickens and boils, about 12 minutes. Transfer to medium bowl. Chill until cold, at least 6 hours. (Can be made 1 week ahead.
Press plastic wrap onto surface of curd and keep chilled.)
Preheat oven to 350°F. Spray bottom of 8-inch-diameter springform pan with nonstick spray. Blend cookie crumbs and butter in small bowl. Press onto bottom of pan. Bake until golden, about 15 minutes. Cool.
Pour 5 tablespoons water into small saucepan. Sprinkle gelatin evenly over. Let stand until gelatin softens, about 15 minutes.
Meanwhile, place 1 3/4 cups lemon curd in large bowl. Stir 3/4 cup curd in another small saucepan over medium-low heat until very warm.
Stir gelatin mixture over medium-low heat until dissolved and liquid is clear (do not boil). Whisk warm gelatin mixture into 3/4 cup warm curd. Gradually whisk gelatin-curd mixture into curd in large bowl. Using electric mixer, beat egg whites in medium bowl until soft peaks form. Gradually add sugar, beating until whites are thick and glossy. Fold whites into curd mixture in 3 additions. Using same beaters, beat cream in another medium bowl until peaks form. Fold into egg white-curd mixture in 3 additions. Pour enough mousse over cooled crust to fill pan completely. Pour remaining mousse into small bowl and reserve. Cover and chill mousse cake, reserved mousse, and remaining curd overnight.
Using long thin knife, cut around cake to loosen. Remove pan sides. Gently spread 3/4 cup of remaining curd over cake. Transfer reserved mousse to pastry bag fitted with small star tip. Pipe rosettes of mousse around top edge of cake. Chill cake until ready to serve. (Can be made up to 8 hours ahead.)
Arrange lemon slices between rosettes. Cut cake into wedges.