Prepare ice bath in large bowl. Juice 1 lemon; reserve juice. Juice additional lemons to yield 1 cup. Make lemon curd: Place 1 cup lemon juice, sugar, egg yolks, whole eggs, and butter in saucepan; whisk to combine. Cook over medium heat, whisking constantly until it begins to boil, about 7 minutes.
Strain curd through fine mesh strainer into a bowl set in an ice bath. Stir periodically until cool; remove from ice bath. Place plastic wrap directly on surface of curd; refrigerate at least 1 hour and up to 3 days.
Place 1 1/2 cups chilled heavy cream in large mixing bowl; whisk until soft peaks form. Reserve 1/2 cup lemon curd for sauce; add remaining lemon curd to whipped cream. Fold gently until well combined.
Place an 8-inch nonstick springform pan on baking sheet. Transfer mousse to springform pan. Place in freezer on baking sheet until firm, at least 4 hours.
Drain candied lemon zest; reserve syrup. Whisk 1/3 cup syrup, reserved 1/2 cup lemon curd, and juice of 1 lemon in a small bowl. Remove frozen mousse from freezer and let warm 4 to 5 minutes. Meanwhile, whisk creme fraiche and remaining 2 tablespoons cream in medium bowl until soft peaks form. Remove sides of springform pan. Slice mousse and place on plates. Spoon lemon sauce around mousse; top with creme fraiche. Garnish with candied zest; serve.
Candied Lemon Zest:
4 lemons, well scrubbed
2 cups sugar
1 cup cool water
Remove zest from lemons with a vegetable peeler, keeping pieces long. Remove white pith using a paring knife. Cut into fine julienne using a very sharp knife. Place julienned zest in a small bowl; cover with boiling water. Let stand 30 minutes; drain.
Bring sugar and the cool water to a boil in a small saucepan over medium-high heat. When sugar is completely dissolved, add julienned zest, reduce heat to medium-low, and cook 10 minutes. Remove from heat, cover, and let stand overnight. Store zest in syrup in an airtight container in the refrigerator for up to 2 weeks.