Heat crushed garlic and 2 tablespoons olive oil in a large saucepan over medium heat until garlic starts to sizzle. Stir in tomatoes, red pepper flakes, oregano, basil, and salt. Bring to a boil, reduce heat to low, and simmer for 30 minutes, stirring occasionally.
Bring a large pot of lightly salted water to a boil. Cook linguine at a boil until tender yet firm to the bite, about 11 minutes; drain.
Heat 2 tablespoons olive oil in a large skillet over high heat. Stir in shrimp and scallops. Cook, stirring frequently, until shrimp turn pink, about 2 minutes. Remove from heat.
Stir shrimp, scallops, mussels, and parsley into tomato mixture. Cook until sauce begins to bubble and mussels open, about 7 minutes. Pour sauce over linguine and serve.