To form the roux, melt 1/3 cup butter in a saucepan over medium heat until a pinch of flour sprinkled in the butter begins to bubble. Whisk in the flour to form a thick paste the consistency of cake frosting. Continue cooking, whisking constantly, until the flour turns peanut butter-brown, about 35 minutes.
Melt 4 tablespoons butter in a small saucepan over medium-high heat. Add celery, onion, bell pepper, and garlic; cook until tender, about 4 minutes. Move half of the vegetables to a large saucepan.
Pour roux into the small saucepan; heat over medium-low. Add diced tomatoes and green chiles, tomato paste, onion powder, garlic powder, white sugar, and cayenne pepper.
Stir in tomato sauce and vegetable oil to the vegetable mixture in the large saucepan. Add roux mixture and 1 cup water. Bring to a simmer; cook 10 minutes. Pour in 2 cups water; bring back to a simmer. Add shrimp and cook until pink, about 3 minutes. Stir in spaghetti, separating the strands. Cover and cook until spaghetti is tender yet firm to the bite, about 10 minutes. Season with salt and black pepper; garnish with parsley.