Heat 1 tablespoon oil in a medium saucepan over medium. Add shrimp shells; cook, stirring often, until aromatic and shells are bright pink, about 1 minute. Stir in broth and 2 cups water; bring to a simmer over medium. Reduce heat to medium-low; cook, undisturbed, until flavors meld, about 15 minutes. Remove from heat. Pour mixture through a fine wire-mesh strainer set over a large heatproof bowl, pressing on solids with a spoon; discard solids. Wipe saucepan clean, and return broth to saucepan. Bring to a simmer over medium-low; cover and keep warm over medium-low.
Grate 3 zucchini on large holes of a box grater placed over a medium bowl; set aside. Cut remaining 2 zucchini into ½-inch cubes; set aside. Heat 2 tablespoons oil in a large skillet over medium-high. Add cubed zucchini; cook, stirring occasionally, until beginning to soften, 2 to 3 minutes. Add grated zucchini; cook, stirring often, until cubed zucchini is tender and liquid released by grated zucchini is mostly evaporated, about 3 minutes. Transfer to a medium bowl; set aside. Wipe skillet clean.
Heat 2 tablespoons olive oil in a large saucepan over medium. Add garlic and crushed red pepper; cook, stirring often, until fragrant, about 1 minute. Add rice, and cook, stirring constantly, 1 minute. Add wine; cook, stirring often, until wine is absorbed, about 1 minute. Add 1 cup warm shrimp broth, and cook, stirring constantly, until broth is mostly absorbed, about 4 minutes. Continue adding broth, ½ cup at a time, stirring constantly until broth is mostly absorbed between additions, until all broth is used up, about 20 minutes total. The risotto is done when rice is al dente (the grains will have firm but tender centers) and suspended in a thick, creamy sauce. Remove from heat. Stir in cheese and cooked zucchini. Season with salt and black pepper to taste. Cover to keep warm.
Heat remaining 1 tablespoon oil in clean skillet over medium-high. Add shrimp, salt, and black pepper. Cook, stirring occasionally, until shrimp are opaque and just cooked through, 3 to 4 minutes. Remove from heat. Stir half of shrimp into risotto.
Divide risotto evenly among 4 serving bowls; top with remaining shrimp. Serve immediately.