Make the saffron rice. Bring the fish stock to a boil in a medium saucepan over medium-high heat and add the sugar, salt, rice vinegar, saffron and rice. Bring back to a boil, reduce the heat to low, cover and simmer for 10 to 15 minutes, or until all of the liquid has been absorbed. Remove from the heat and let sit, covered, for 10 minutes, then stir in the Parmesan, oil and butter.
Toast the nori sheets. Holding the nori sheets 4 to 5 inches above low heat using metal tongs, move the nori back and forth slowly and continuously until the entire sheet changes color from dark green to lighter green, and it crisps slightly. Cool to room temperature and cut the nori sheets into 1 ¼- by 2-inch rectangles.
Make the sushi squares. Press 3 ½ cups of the rice mixture firmly and evenly into the bottom of a plastic wrap-lined 6-inch-square baking pan, adjusting the pressure until the rice is about 1 ¼ inches thick throughout the pan. Place a small cutting board over the baking pan. Turn the pan and board over and tap the pan gently to rlease the rice, then remove the pan. Use a hot, wet knife to score the surface of the formed rice in three 2-inch-wide rows in each direction, creating a grid of 9 squares of equal size. Using the grid, cut the formed rice into squares, cleaning and wetting the knife before each slice. Separate the rice squares and flatten any rough sides. Adhere one nori strip to each side of each rice square. Reserve until needed.
Make the sauce. Combine the shallots, white wine, and white wine vinegar in a small saucepan and bring to a boil over medium-high heat. Boil until the liquid is reduced to 2 tablespoons, about 15 minutes. Add the cream and bring to a boil over high heat. Reduce the heat to medium high and continue to cook uncovered for 7 to 8 minutes, or until the sauce is reduced to about 1 ½ cups. Add the saffron powder and remove the sauce from the heat. Season to taste with the salt and white pepper.
Prepare the topping. Shuck the mussels. Fry them quickly in a large sauté pan with 2 tablespoons of vegetable oil over medium-high heat until they change color and begin to firm up, about 1 minute. Set aside.
Blanche the scallops and shrimp. Bring 2 quarts of water and the salt to a boil over medium-high heat in a medium saucepan. Add the scallops and cook for 2 minutes, or until they lose their translucency and turn opaque. Drain and set aside, return the cooking liquid to the pan, and return to a boil. Add the shrimp and cook for 30 seconds, or until they turn pink but just before they begin to curl.
Whisk together the soy sauce and olive oil in a small bowl.
Drizzle 2 ½ tablespoons of the saffron sauce and 1 tablespoon of the vinaigrette around each plate. Place a sushi square in the center of the plate and garnish with 2 pieces of the scallops, 3 of the mussels, 3 to 4 pieces of the shrimp, 1 teaspoon of the salmon roe, and ¼ teaspoon of the pink peppercorns.