Bring the chicken stock to a boil in a medium saucepan. Add the crayfish a little at a time so as not to stop the boil, and let cook until pink. Using a slotted spoon remove the crayfish and set aside until they are cool enough to handle. (Skim off and discard any impurities that have risen to the surface of the stock.) Now, remove the shells from the shrimp and add the shells to the pot of chicken stock, reserving the uncooked shrimp on a utility platter. Allow the shrimp shells to cook in the broth until pink, then strain the stock into another container and discard the shells.
While the shrimp shells are boiling, heat the olive oil in a saute pan over medium-high heat. Brown the sausage and remove to a platter until it is cool enough to handle. In the sausage drippings over medium heat, saute the onion, celery, and green bell pepper until the onion turns translucent. Reduce the heat and whisk in the flour to make a roux, and let cook for about 15 minutes until it becomes a brown roux.
While the roux is browning, slice the sausage into 1/4 to 1/2-inch pieces. Put the tomato paste in a small bowl and add some of the warm stock to make a slurry. Gradually whisk the warm stock and the tomato paste into the roux. Add the garlic powder, crushed red pepper, okra, 1 tablespoon parsley, scallions, red bell pepper, tomatoes and sausage. Cook over low heat until the okra - which contains a thickening agent - is tender, about 15 minutes, and the sausage is cooked through. While the mixture is cooking, remove the crayfish from their shells. (This is done by cracking the tail with both hands and forcing it back out through the curve of the tail.) Add the oysters and shrimp to the pot and cook about 5 minutes until the oysters are plump and the shrimp are pink. Stir in crayfish. Remove pot from heat and carefully fold in crab, trying not to break up the lumps.
To serve, place some rice on serving dish and spoon some gumbo over. Sprinkle with file powder and garnish with remaining parsley.