Sprinkle ½ tsp gelatin over 2 Tbsp water in small bowl and let sit until gelatin softens, about 5 minutes. Repeat with second small bowl, remaining ½ tsp gelatin, and remaining 2 Tbsp water.
Mix together 2 eggs and 3 yolks together in medium saucepan until thoroughly combined
Mix in 1 cup sugar, cornstarch, and salt until well combined.
Whisk in lemon zest and juice and heavy cream.
Cook over medium-low heat, stirring constantly, until thickened and slightly translucent, 4 to 5 minutes (mixture should register 170 degrees).
Stir in 1 water-gelatin mixture until dissolved. Remove pan from heat and let stand for 2 minutes.
Remove 1¼ cups curd from pan and pour through fine-mesh strainer set in bowl.
Transfer strained curd to prepared pie shell.
Place filled pie shell in freezer.
Add remaining water-gelatin mixture and cream cheese to remaining curd in pan and whisk to combine. (If cream cheese does not melt, briefly return pan to low heat.) Pour through strainer into now-empty bowl.
Whip 3 egg whites on medium-low speed until foamy, about 2 minutes.
Increase speed to medium-high and slowly add remaining ¼ cup sugar.
Continue whipping until whites are stiff and glossy, about 4 minutes.
Add curd–cream cheese mixture and whip on medium speed until few streaks remain, about 30 seconds, mix until combined and there are no streaks.
Remove pie shell from freezer and carefully pour chiffon over curd, allowing chiffon to mound slightly in center. Refrigerate for at least 4 hours or up to 2 days before serving