Preheat oven to 350°F and position a rack in the center.
Make the cream sauce by melting 4 tablespoons butter over medium heat in a medium saucepan. Whisk in the flour and cook for about 2 minutes until lightly toasted.
Slowly add the clam juice and milk, whisking constantly. Season with 1 1/2 teaspoons kosher salt and 1/4 teaspoon black pepper. Bring to a boil, then reduce heat and simmer until thickened, about 8-10 minutes. Adjust seasoning as needed.
Cook lasagna noodles in salted boiling water per package instructions until al dente. Drain and rinse under cool water, then drain again.
Lay noodles in a single layer on a baking sheet. Lightly coat with olive oil and overlap slightly if necessary.
Chop parsley leaves until you have 1/4 cup for the sauce and reserve 1 tablespoon for garnish. Juice the lemon to yield 1 tablespoon juice. Finely chop garlic.
In a large skillet, melt 4 tablespoons butter over medium heat. Add garlic and cook until golden, about 3 minutes.
Add white wine and boil until almost evaporated, about 3 minutes. Stir in shrimp and cook until pink and just cooked through, about 4 minutes.
Remove skillet from heat. Stir in 1/4 cup parsley, lemon juice, and 1/4 teaspoon kosher salt. Adjust seasoning as needed.
In a medium bowl, combine ricotta cheese, Parmesan, remaining 1 teaspoon kosher salt, and 1/4 teaspoon black pepper. Adjust seasoning as needed.
Assemble the lasagna: Spread 1/2 cup cream sauce in the bottom of the baking dish. Layer 1/3 of the noodles over the sauce.
Spread half the ricotta mixture over noodles, then evenly layer half the shrimp mixture, half the lobster, and half the crab meat. Sprinkle with 1 cup mozzarella.
Dollop and spread 1 cup cream sauce over cheese. Repeat layering with half of remaining noodles, remaining ricotta mixture, remaining shrimp mixture, lobster, crab, and mozzarella.
Top with remaining noodles, spread remaining cream sauce, and sprinkle with remaining 2 cups mozzarella.
Coat a sheet of aluminum foil with cooking spray and loosely cover the baking dish spray side down.
Bake covered for 30 minutes. Remove foil and bake uncovered for 20-25 minutes until golden and bubbly.
Let cool at least 15 minutes before slicing. Garnish with reserved parsley and serve.