Add raspberries, sugar, and lemon juice to a small saucepan, bring to a boil over medium-high heat while stirring constantly, then reduce heat and simmer for 10 to 15 minutes until thickened.
Remove the raspberry sauce from heat, strain through a sieve to remove seeds, and set aside to cool completely.
Line the bottom of an 8-inch (20 cm) springform pan with parchment paper, then line the sides with an acetate sheet.
Crush Oreo cookies in a food processor until fine crumbs form, add melted butter, and mix until evenly moistened.
Press the cookie mixture firmly into the bottom of the pan with the back of a spoon and set aside.
Place white chocolate and 4 tbsp cream in a heatproof bowl, set over simmering water, and stir gently until melted and smooth.
Sprinkle gelatin over cold water in a small bowl, let it bloom for 5 to 10 minutes, then heat gently until dissolved and pour into the melted chocolate, allowing it to cool slightly.
Beat cream cheese in a large bowl until smooth, add powdered sugar and vanilla, then mix in mango puree and the melted chocolate mixture until combined.
Whip the chilled cream in a separate bowl until stiff peaks form, then gently fold it into the mango cream cheese mixture.
Spread one-third of the filling over the crust, spoon half the raspberry sauce on top, and use a skewer or knife to create swirls.
Spread another third of the filling, spoon the remaining raspberry sauce on top, and swirl again.
Spread the last third of filling, smooth the surface, cover with plastic wrap, and refrigerate for at least 4 to 6 hours or preferably overnight.
Crush the Oreo cookies for the coating and press them around the sides and top of the chilled cake until evenly covered.
Place marshmallows in a heatproof bowl, microwave for 1 minute until melted, then use two forks to stretch thin strands and drape them over the cake like spider webs.