For the calamari: Combine the calamari and buttermilk in a large glass container. Cover and refrigerate for 30 minutes. Drain and rinse. Transfer to a mixing bowl and set aside.
For the Venetian vinaigrette: Combine the vinegar, garlic, lemon zest and juice in a medium mixing bowl. Whisk in the oil slowly to emulsify. Once thickened, whisk in the parsley, cilantro, oregano and scallions. Season with salt and pepper. Refrigerate in an airtight container until ready for use (or up to 48 hours).
For the court bouillon: Place 12 cups cold water, the celery, carrot, onion, parsley, thyme, bay leaves, chili, lemon and black peppercorns into a large stockpot fitted with a fine mesh colander attachment. Bring to a boil and reduce to a simmer. Poach by placing the calamari and the shrimp into the colander and simmering until the calamari is firm and the shrimp is pink, about 3 minutes. Remove to a mixing bowl and set aside. Poach the clams until the shells open, 4 to 6 minutes.
Transfer the clams to the already poached shellfish, discarding any unopened shells. Add the sliced tomato to the shellfish. Toss with two-thirds of the vinaigrette, season and refrigerate until chilled, about 10 minutes.
For the salad: Gently toss the arugula and fennel in a large mixing bowl with the remaining vinaigrette to coat. Season with salt and pepper. Transfer to a large serving dish. Arrange the marinated seafood atop the salad. Garnish with the diced tomatoes, scallions, lemon wedges and fennel fronds around the platter.