Make the crepes: In a bowl place the flour, whisk in the milk and water, a little at a time. Mix in the eggs, the salt and butter and continue to whisk until smooth. Let stand, covered, 30 minutes. Heat a crepe pan over moderate heat until very hot, brush with melted butter. Add 1/4 cup of batter to center of pan. Tilt pan in all directions, pouring out excess batter into bowl. Cook 30 seconds, or until golden brown. Turn and cook 20 to 30 seconds more. Invert crepe onto plate. Continue to make crepes in the same manner.
Make the filling: In a skillet, set over moderate heat, melt the butter and cook the shallots until softened. Add the shellfish and cook for 2 minutes. Add the wine and Madeira, simmer, covered, 1 minute. Uncover, increase heat to high and reduce until all the liquid has been evaporated. Season with salt and pepper. Stir in chives.
Make the white sauce: In a saucepan set over moderately low heat, melt the butter, add the flour and cook, whisking, for 2 minutes. Add the hot milk and salt and pepper to taste, whisking. Bring the liquid to a boil, whisking, and simmer 1 minute. In a small bowl combine the egg yolk and cream. Remove the pan from the heat and whisk in the yolk mixture. Return to heat and cook, stirring, until very thick. Season with nutmeg, salt and pepper. Fold in shellfish and cheese.
Make the lobster sauce: In a saucepan set over moderately low heat melt the butter, add the flour and cook, whisking, for 2 minutes. Add the hot lobster stock and salt and pepper to taste, whisking. Bring the liquid to a boil, whisking, and simmer until slightly thickened. Add heavy cream and 3 tablespoons of the lobster butter and simmer, whisking occasionally, for one hour. Whisk in remaining lobster butter.
To assemble the crepes: Arrange crepe, browned-side down, on counter. Place 1/2 cup filling in center. Fold 2 sides of crepe over mixture so that they meet in the center. Fold top and bottom sides of crepe over so that they meet in the center. Turn crepe over and transfer to a buttered flameproof baking dish. Prepare remaining crepes in the same manner. Cover the crepes with some of the lobster sauce and sprinkle with grated Swiss cheese. Put under a preheated broiler 1 to 2 minutes, or until cheese is melted.
Remove meat from the lobster, and reserve. Add shells to the hot lobster stock or fish broth, and simmer for 1 hour. Remove shells, and chop into small pieces. Add shells to food processor work bowl (or blender) and add butter. Pulse 5 to 6 times. Return mixture to saucepan and melt butter again. Strain carefully and reserve lobster butter.