Mix sugar, water, vinegar, and honey together in a bowl. Add mango cubes; stir to coat. Cover bowl with plastic wrap and refrigerate for 8 hours to overnight.
Drain liquid from mango, reserving 1/4 cup of the liquid.
Heat olive oil into a pot over medium heat; cook and stir shallot and ginger until slightly tender, about 5 minutes. Add rice; cook and stir until slightly toasted, 1 to 2 minutes. Pour chicken broth and coconut milk over rice; bring to a boil. Cover pot, reduce heat to medium-low, and simmer until rice is tender and liquid is absorbed, about 20 minutes.
Stir mango, 1/4 cup reserved vinegar mixture, and wine together in a skillet over medium heat; cook and stir until warmed, about 2 minutes. Add spinach and cook until wilted, about 1 minute. Stir rice mixture and Parmesan cheese into mango mixture; remove from heat and let sit until liquid absorbs and cheese melted, about 7 minutes.
Place 1 watermelon slice on a serving platter; top with risotto. Place the 2nd watermelon slice atop risotto to garnish.