Preheat the oven to 400°F.
Prep. The following day, remove as much of the solidified fat as you can from the surface of the ragout, reserving about ¼ cup. Let the ragout return to room temperature, about 1 hour. 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.
Make the paste. Bring the ragout to a simmer on the stove top. Cut the refrigerated, boiled 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 this paste into the ragout and simmer over low heat for 15 minutes, stirring occasionally. Stir in all the cooked meats.
Roast the duck legs. 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°F.
Layer the cassoulet. Untie and unroll the fat back rolls. Line the bottom of a 5- to 6-quart earthenware casserole with the fat back. Using a large, slotted spoon, transfer half of the ragout to the earthenware casserole. Top with the torn pieces of duck confit in an even layer, then cover with the remaining ragout. Add the remaining 2 cups of broth to the cooking liquid in the cast-iron casserole and season lightly with salt and pepper.
Bake. Pour the liquid over the ragout and drizzle with 2 tablespoons of the reserved skimmed fat. Bake the cassoulet for 1½ hours.
Cook the sausages. 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 chunks.
Add sausage and topping. Reduce the oven temperature to 275°F. Gently stir in the skin that has formed on the cassoulet. Nestle in the sausage chunks and drizzle with the remaining 2 tablespoons of reserved fat. Sprinkle with the breadcrumbs.
Bake, rest, and serve. Bake the cassoulet for 1 hour longer, until it is bubbling around the edges and richly browned on the surface. Transfer to a wooden board or towel-lined cooling rack and allow to rest for at least 25 minutes before serving.