Melt 6 tablespoons butter in a Dutch oven over low heat; add shallots, and sauté 1 minute. Increase heat to high. Add mushrooms and lemon juice, and cook, stirring constantly, until mushrooms are just tender. Transfer to a bowl.
Peel shrimp; devein, if desired.
Melt 4 tablespoons butter in Dutch oven over medium heat; add shrimp, and saute 3 to 5 minutes or just until shrimp turn pink. Add to mushroom mixture.
Bring scallops and half-and-half to a boil in a saucepan over medium-high heat. Reduce heat, and simmer, stirring often, 3 to 5 minutes or until scallops are opaque. Pour through a wire-mesh strainer into a small bowl; add scallops to mushroom mixture. Reserve half-and-half.
Drain any liquid from mushroom mixture. Add liquid to reserved half-and-half.
Melt remaining 6 tablespoons butter in Dutch oven over low heat; whisk in flour until smooth. Cook, whisking constantly, 1 minute. Gradually add half-and-half mixture; cook over medium heat, whisking constantly, until mixture is thickened and bubbly. Add Parmesan cheese and next 5 ingredients; cook, whisking constantly, 3 minutes or until cheese melts and sauce is smooth. Remove from heat; stir in mushroom mixture.
Spoon into 10 lightly greased shell-shaped baking dishes or individual serving bowls, and sprinkle evenly with shredded Swiss cheese. Place on 2 (15- x 10-inch) jellyroll pans.
Broil 5 1/2 inches from heat (with electric oven door partially open) 8 minutes or until Swiss cheese is golden and mixture is bubbly. Serve immediately.
Note: Mixture may be prepared a day ahead and chilled. Broil just before serving.