Preheat oven to 325°F. Oven temperatures may vary. In a large pot of salted boiling water and 1 tablespoon of olive oil cook shells according to package instructions for baked shells. (It is usually 3 minutes less than the package's recommended cooking time.) Transfer shells to a large bowl of cold water until cooled slightly, then drain. (Reserve pasta-cooking water for next step.)
Poach shrimp in same pot of pasta water until just barely cooked through, about 2 minutes. Drain and chop shrimp into small pieces.
In a mixing bowl, combine crabmeat with shrimp, scallops, Dijon mustard, mayonnaise, Old Bay seasoning, shallots, parmesan cheese and parsley. Mix well and season with salt and pepper.
In a small saucepan, melt 3 tablespoons butter over medium-high heat (do not allow it to brown). Add flour and whisk to form a paste. Continue to cook, stirring, until raw flour scent is gone, about 1 minute. Whisking constantly, add milk in a thin, steady stream, or in increments of a couple of tablespoons at a time, whisking thoroughly and getting into all corners of pan to maintain a homogeneous texture. The sauce will initially be very thick, then get very thin once all the milk is added. Add bay leaves and allspice.
Heat, stirring, until sauce comes to a simmer and begins to thicken slightly. Reduce heat to low and cook, stirring, until sauce is thick enough to lightly coat the back of a wooden spoon, about 3 minutes. Season with salt and pepper.
Grease a 9- by 13-inch baking dish with oil. Spread about 1 cup béchamel sauce on bottom of baking dish evenly. Using a spoon, fill a shell with a large scoop of seafood mixture and place in baking dish with the opening side up. Repeat until baking dish is full.
Spoon remaining béchamel sauce on top of shells. Melt remaining 2 tablespoons butter and toss with bread crumbs in a small bowl, season with salt. Sprinkle bread crumbs all over shells.
Bake shells until heated through, about 35 minutes. Switch oven to broiler setting and, watching very closely to prevent burning, broil until bread crumbs turn golden on top, rotating baking dish occasionally for even browning. Scatter minced parsley on top for garnish and serve. Smacznego!