Cook garlic, onions, bay leaf, oregano, and red pepper flakes with salt and pepper in oil in an 8-quart heavy pot over moderate heat, stirring, until onions are softened, about 5 minutes. Stir in bell pepper and tomato paste and cook, stirring, 1 minute. Add wine and boil until reduced by about half, 5 to 6 minutes. Add tomatoes with their juice, clam juice, and broth and simmer, covered, 30 minutes. Season with salt and pepper.
While stew is simmering, hack crab leg through shell into 2- to 3-inch pieces with a large heavy knife. Add crab pieces and clams to stew and simmer, covered, until clams just open, 5 to 10 minutes, checking every minute after 5 minutes and transferring opened clams to a bowl with tongs or a slotted spoon. (Discard any unopened clams after 10 minutes.) Lightly season fish fillets, shrimp, and scallops with salt and add to stew, then simmer, covered, until just cooked through, about 5 minutes. Discard bay leaf, then return clams to pot and gently stir in parsley and basil.