Cook the Pasta Shells: Preheat the oven to 325°F (160°C). Boil jumbo shells in salted water according to the package instructions for baked shells (or cook 3 minutes less than usual). Drain and cool in a bowl of cold water. Drizzle lightly with olive oil to prevent sticking and set aside.
Poach the Shrimp: In the same pot of pasta water, poach the shrimp for 1-2 minutes, until just cooked through. Transfer to an ice bath, then drain and chop into small pieces.
Make the Filling: In a mixing bowl, combine the crabmeat, chopped shrimp, scallops, Dijon mustard, mayonnaise, Old Bay seasoning, minced shallot, and parsley. Mix well and season with salt and pepper.
Prepare the Béchamel Sauce: In a small saucepan, melt 1 1/2 tbsp butter over medium-high heat. Add flour and whisk to form a paste. Cook for about 1 minute to remove the raw flour taste. Slowly add milk while whisking constantly, and add the bay leaf. Cook until the sauce thickens enough to coat the back of a spoon (about 3 minutes). Season with salt and pepper. Remove the bay leaf and set aside.
Assemble the Shells: Lightly grease a 9x13-inch baking dish with olive oil. Spread 1/2 cup of béchamel sauce across the bottom of the dish. Fill each shell with the seafood mixture and arrange them in the baking dish, open side up. Spoon the remaining béchamel sauce over the stuffed shells.
Add the Breadcrumb Topping: Melt the remaining 2 tbsp butter and mix with the panko breadcrumbs. Sprinkle the breadcrumb mixture over the shells.
Bake and Broil: Bake the shells for about 25 minutes, or until heated through. Switch the oven to broiler mode and broil until the breadcrumbs are golden brown, rotating the dish for even browning. Watch closely to avoid burning.
Serve: Garnish with minced parsley and serve hot.