Bring clam juice and smashed garlic to a simmer in a large pot. Add mussels (or clams) and cook, covered, until open, 3 to 5 minutes. Discard garlic; remove mussels from the pot, discarding any that did not open. Using a fine-mesh sieve lined with cheesecloth, strain broth into a bowl; set aside. Wipe out the pot.
Add 1 tablespoon oil to the pot and heat over medium-low heat. Add onion and minced garlic and cook, stirring, until softened, 5 to 7 minutes. Increase heat to high; add squid and cook, stirring, until the squid turns opaque, about 1 minute. Add wine and 1/3 cup vinegar; cook, stirring occasionally, until the liquid has reduced by half, 5 to 7 minutes.
Add tomatoes and the reserved mussel broth. Reduce heat to low, cover and simmer until the squid is tender, 18 to 20 minutes.
Just before serving, add fish and shrimp to the simmering stew and cook, covered, just until the fish is opaque in the center, 3 to 5 minutes. Stir in the reserved mussels and heat through, about 1 minute. With a slotted spoon, remove solids to a bowl and cover to keep warm. Stir 3 tablespoons parsley, breadcrumbs and the remaining 1 tablespoon oil into the sauce. Simmer until the sauce thickens to the consistency of heavy cream, 1 to 2 minutes. Adjust seasoning with pepper and vinegar. Return the solids to the sauce. Serve immediately, garnished with the remaining 1 tablespoon parsley.
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