You'll need 8 hours for chilling, so your best bet is to make this the night before and let it chill until the next evening.
In a medium saucepan, whisk the eggs, 1 cup sugar, the lemon zest and juice, and the salt. Add the butter and stir over medium heat until the mixture is thick enough to coat the back of a spoon, about 6 minutes. Transfer to a medium bowl. Cover the lemon curd with plastic wrap, pressing the wrap directly on top of the lemon curd. Refrigerate until cold, about 3 hours.
In a small saucepan, stir 1 1/2 cups water and 3 tbsp. sugar until the sugar dissolves, about 2 minutes. Remove from heat. Stir in the elderflower liqueur. Let the elderflower syrup cool.
Using an electric mixer, in a large bowl, beat the cream cheese and the remaining 2 tbsp. sugar on medium speed until smooth, 1 to 2 minutes. Add the cream and beat on high speed until soft peaks form, 1 to 2 minutes. Fold in the lemon curd.
Working with a handful at a time, dip the cookies in the elderflower syrup and turn until coated. Arrange the cookies in a single layer on the bottom of two 1 1/2- to 2-quart wide, shallow baking dishes, covering the bottom of each dish. Spoon 1 1/2 cups of the lemon cream over the cookies in each dish. Dip the remaining cookies in the remaining elderflower syrup. Arrange the cookies on top of the lemon cream in each dish. Spoon the remaining lemon cream on top, about 1 1/2 cups for each dish. Smooth the tops. Cover the icebox cakes with plastic wrap and refrigerate overnight.
Preheat the oven to 500°. For the meringue, using an electric mixer, in a large bowl, beat the egg whites and cream of tartar at medium speed until soft peaks form, about 1 minute. Gradually beat in the sugar. Increase the speed to high and beat until stiff peaks form, 1 to 2 minutes more. Uncover the icebox cakes and spread the meringue on top, dividing equally, to form swirls and peaks. Bake until the meringue is browned in spots, 3 to 5 minutes.
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