Melt 3 tablespoons butter in heavy saucepan over low heat. Add flour, stirring until smooth; cook 1 minute stirring constantly. Gradually add half and half; cook over medium heat, stirring constantly until mixture is thickened and bubbly. Stir in salt and white pepper; set aside.
Melt remaining 3 tablespoons butter in large skillet over medium heat. Add scallops and green onions; cook 3 minutes, stirring constantly. Add mushrooms and cook 3 minutes more.
Stir cream sauce, crabmeat, and wine into the scallop mixture. Cook just until thoroughly heated.
To serve, spoon about 1 tablespoon seafood mixture into each shell.