Put the ham hocks, pork shoulder cubes and skin in a large dish; season lightly with salt and pepper. Cover and refrigerate overnight. In a bowl, cover the beans with 3 inches of water and soak overnight.
The next day, in a medium saucepan, cover the salt pork and the seasoned skin with water. Bring to a boil, then simmer over moderate heat until the skin is supple, about 30 minutes. Drain and cool. Refrigerate the salt pork. Cut the pork skin into 5 long pieces, roll each piece into a bundle and tie with string.
Dry the ham hocks and pork shoulder cubes with a paper towel. In a very large, enameled cast-iron casserole, heat the duck fat. Add half of the pork cubes and cook over moderately high heat until lightly browned all over; transfer to a plate. Repeat with the remaining pork cubes. Add the ham hocks to the casserole and brown them lightly. Add the carrots and onions and cook over moderate heat, stirring occasionally, until the onions are golden, about 7 minutes. Add the pancetta and brown it lightly. Add the prosciutto, the head of garlic and the tomato and cook, stirring, for 1 minute. Add 2 quarts of the broth, the bouquet garni, pork skin bundles and the browned pork and its juices and bring to a boil. Cover the casserole and gently simmer the ragout over low heat for 1 1/2 hours, stirring occasionally.
Drain the beans. In a large saucepan, cover the beans with water and bring to a boil over moderate heat. Simmer the beans for 3 minutes, then drain. Add the beans to the ragout and simmer until the beans are just tender, about 2 hours. Let the ragout cool, then refrigerate overnight.
Remove as much of the solidified fat as you can from the surface of the ragout; reserve 1/4 cup of the fat. Let the ragout return to room temperature. Pick out the ham hocks, pancetta and prosciutto. Cut the meats into bite-size pieces; discard the bones, skin and gristle. Pick out the pork skin bundles and the head of garlic and reserve. Discard the bouquet garni.
Preheat the oven to 400°. Bring the ragout to a simmer. Cut the blanched salt pork into small pieces. Squeeze the cooked garlic cloves into a food processor. Add the salt pork and the raw garlic cloves and process to a smooth paste. Stir the paste into the ragout and simmer over low heat for 15 minutes, stirring occasionally. Stir in all of the cooked and cured meats.
7. Meanwhile, arrange the duck confit legs in a baking dish and roast just until heated through, about 15 minutes. Remove the meat from the bones in large pieces. Cut the skin into strips. Discard the bones.
Turn the oven down to 325°. Untie and unroll the pork skin bundles. Line the bottom of a 5- to 6-quart earthenware casserole with the pork skin, fat side down. Using a large slotted spoon, transfer half of the ragout to the earthenware casserole. Top with the duck confit in an even layer, then cover with the rest of the ragout. Add the remaining 2 cups of broth to the cooking liquid in the cast-iron casserole and season lightly with salt and pepper. Pour the liquid over the ragout and drizzle with 2 tablespoons of the reserved skimmed fat. Bake the cassoulet for 1 1/2 hours.
Heat the vegetable oil in a medium skillet. Add the sausages and cook over moderately high heat until browned all over. Let cool, then cut the sausages into 3-inch pieces.
10. Reduce the oven temperature to 275°. Gently stir in the skin that has formed on the cassoulet. Nestle in the sausages and drizzle with the remaining 2 tablespoons of reserved fat. Sprinkle with the bread crumbs. Bake the cassoulet for 1 hour longer, until it is richly browned on the surface. Transfer to a cloth-lined rack and let rest for at least 20 minutes before serving.