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.
The nutrition data for this recipe includes the full amount of the vinegar mixture ingredients. The actual amount of the vinegar mixture consumed will vary.