Bring a large pot of salted water to a boil. Cook the spaghetti until al dente. Reserve 1 ½ cups of pasta water, then drain and set aside.
In a large sauté pan over medium-high heat, warm the olive oil. Add the garlic and red pepper flakes, cooking until fragrant, about 30 seconds.
Pour in the dry white wine and let it cook for 2 minutes to reduce slightly. Stir in the crushed tomatoes, salt, black pepper, oregano, Calabrian chili paste (if using), and fresh basil. Reduce heat to low and let the sauce simmer for 10 minutes, stirring occasionally.
Nestle the littleneck clams and mussels into the sauce, cover, and cook for 5-7-8 minutes, or until the shells open. Discard any that don’t open. Season the shrimp and calamari with salt and pepper, then add them to the pan. Cook for 2-3 minutes until the shrimp turn pink and the calamari is tender.
Add the cooked spaghetti to the sauce, tossing to coat. Stir in the butter and add reserved pasta water a little at a time, as needed, to loosen the sauce.
Transfer the pasta to a large serving bowl, ensuring the seafood is evenly distributed. Garnish with fresh parsley and a squeeze of lemon juice. Serve with freshly grated Parmigiano Reggiano, if desired.