Insert the base of a 9-in. springform pan lip-side down. Lightly spray bottom and sides with oil. Line bottom with parchment, and the sides with a long strip of parchment, about 3 in. wide.
Beat sugar with 1/2 cup butter, eggs, yolks and salt in a medium saucepan with an electric mixer on high, until creamy, about 1 min. Beat in lemon juice. (Mixture may curdle but will smooth out during cooking.) Set saucepan over medium-high. Cook, whisking constantly, until curd is thickened and coats the back of a spoon, 3 to 5 min. Scrape into a large shallow dish and cover with plastic wrap. Chill in freezer until cool, about 40 min.
Melt 1/3 cup butter in a medium saucepan set over medium-high. Stir until butter turns foamy. Stir in graham-cracker crumbs until toasted and golden, about 2 min. Pour crumbs into prepared pan and press down firmly to form an even crust. Freeze for 20 min.
Beat mascarpone and cream cheese in a large bowl with electric mixer on high until fluffy, 3 to 4 min. Beat in 2 cups of cold lemon curd. Mix until combined, then pour over frozen crust. Smooth top. Freeze until top is firm to the touch, about 30 min. Spread remaining lemon curd over the top of the cheesecake and freeze for 6 hours or overnight. Let stand at room temperature for 1 hour before serving.