Toss 2 cups plums, 1 tablespoon honey and a pinch of salt together in a medium bowl. Cover and chill until ready to serve.
Heat 3/4 cup honey and 1/4 cup water in a large saucepan over medium high until boiling. Add remaining plums and cook until plums begin to fall apart and mixture is jammy, 15 to 20 minutes. Let cool slightly; transfer to a blender. Puree plum mixture, yogurt, cardamom, gin and 1/8 teaspoon salt until completely smooth. Line a medium metal bowl or loaf pan with plastic wrap and scrape in sorbet mixture. Freeze until firm, about 6 hours.
Meanwhile, arrange a rack in center of oven; preheat to 350 degrees F. Toss pistachios, pine nuts, 1/4 cup honey, and 1/2 teaspoon salt in a medium bowl. Scrape onto a silpat- or parchment-lined rimmed baking sheet and spread in a single layer. Bake brittle until pistachios are golden brown and honey is bubbly, 15–20 minutes. Sprinkle with rose petals and lightly press to adhere. Let cool until hardened, then break up into small pieces; discard any dark brown, bitter pieces.
Scoop sorbet into bowls and top with macerated plums and broken brittle.