Fill a large pot with 3 quarts of water and add the Old Bay seasoning and 1 tablespoon of salt. Bring to a boil, add the shrimp, lower the heat, and simmer for 3 minutes, until just firm. With a skimmer or slotted spoon, transfer the shrimp to a large bowl. Leave 2 cups of the poaching liquid in the pot and discard the rest.
Add the wine to the poaching liquid and bring to a boil. Add the scallops, lower the heat, and simmer for 2 minutes, until just cooked. With the skimmer, transfer the scallops to the bowl with the shrimp. Bring the poaching liquid back to a boil, add the calamari, and simmer for 2 to 3 minutes, until just cooked. Be careful not to overcook any of the seafood or it will be tough! With the skimmer, transfer the calamari to the bowl.
Bring the poaching liquid to a boil again, add the mussels, cover, and simmer for 4 to 5 minutes, until all the shells have opened, discarding any that don't open. Turn off the heat and set aside until the mussels in the broth are cool enough to handle. Remove the mussels from the shells and add to the bowl. Add 12 of the shells to the seafood and discard the rest. Set aside 1/2 cup of the poaching liquid, discarding the rest. Drain the seafood in a colander and put it all back into the bowl.
For the dressing, heat the olive oil in a medium (10-inch) saute pan over medium heat. Add the garlic, oregano, and red pepper flakes and cook for 1 minute. (Be careful: Overcooked garlic will be bitter.) Add the tomatoes and cook over medium heat for 2 more minutes. Add the reserved poaching liquid, the limoncello, lemon zest, lemon juice, 1 tablespoon salt, and 1 teaspoon pepper and cook for 1 more minute. Pour the sauce over the seafood and toss gently. Add the fennel and parsley. Cut a lemon in half lengthwise, cut it thinly crosswise, and add it to the salad. Toss gently to combine and cover with plastic wrap. Chill for at least 3 hours or overnight.
To serve, sprinkle with 2 teaspoons salt, 1 teaspoon pepper, and the juice of the remaining lemon. Taste for seasonings and serve cold or at room temperature.