Stir together graham cracker crumbs, melted butter, and sugar until the texture of wet sand. Press firmly into the bottom of a 9-inch springform pan, going about 1 inch up the sides. Bake at 350°F for 8 minutes until set and fragrant. Cool completely on a wire rack, about 20 minutes.

Puree the cubed mangoes in a blender until completely smooth, scraping down the sides as needed. You need 1.5 cups puree; reserve any extra for serving. Set aside.

Sprinkle gelatin over cold water in a small bowl. Let stand 5 minutes until spongy, then microwave for 10 seconds until liquid and clear. Whisk immediately into the warm mango puree until fully combined.

Beat heavy cream and powdered sugar in a large bowl with an electric mixer on medium-high speed until stiff peaks form, about 3 minutes. Be careful not to overbeat. Transfer to another bowl and refrigerate.

In the same bowl (no need to clean), beat softened cream cheese and lemon juice on medium speed until smooth and fluffy, about 2 minutes. Scrape down the bowl twice. The mixture should be lump-free.

With mixer on low, gradually add the mango-gelatin mixture to the cream cheese, beating until fully incorporated and smooth. Gently fold in half the whipped cream to lighten, then fold in the remaining cream until no streaks remain. Work quickly before the gelatin sets.

Pour the mousse over the cooled crust, smoothing the top with an offset spatula. Tap the pan gently on the counter to release air bubbles. Refrigerate uncovered for at least 4 hours, preferably overnight, until completely set and firm to the touch.

Run a thin knife around the edge of the pan before releasing the springform. Top with fresh mango slices in a spiral pattern and sprinkle with toasted coconut if desired. Slice with a warm, clean knife for clean cuts.