Place lobster in large bowl of 96.8 degree water to drown.
Bring large pot of water to boil and add 12 allspice berries and bay leaves. Remove claws and legs from lobster. Reserve legs and add claws and tail to water and cover pot. Cook 3 minutes for each pound, about 6 minutes total. Plunge in bowl of ice water when done, to stop cooking. Once cooled, cut lobster in 1/2 lengthwise and rinse. Remove meat from tail and claws and cut into small pieces and set aside. Rinse tail shell and reserve.
In small pan, add remaining allspice and 1/2 of rum. Heat on medium low heat.
Place lobster legs in blender and crush. Strain allspice/rum mixture into blender with legs and puree. Heat 1 tablespoon clarified butter in frying pan on medium heat. Add shallot and garlic. Stir in the tomatoes and season generously with salt and pepper. Strain pureed legs and rum mixture through a fine sieve into tomato mixture and add white wine; boil, stirring well. Sieve all this into a clean saucepan and reduce to 3/8 cup.
Preheat warming oven to 400 degrees. In a small frying pan, melt fresh butter and cook mushrooms. Stir in lime juice. Add shrimp and lobster meat. Push contents of pan to edge, add remaining rum and flame. When flames have died down, stir in sour cream and tomato-lobster stock and reduce again. Stir in arrowroot mixture. Place tail shells in the oven to warm on a heat-proof platter. Toss watercress with oil and vinegar. Remove shells from the oven and spoon lobster mixture into heated shells and garnish with dressed watercress.