For brine: Bring first 5 ingredients and 8 cups water to a simmer in a medium saucepan over medium heat, stirring until sugar and salt dissolve. Remove from heat; let cool to room temperature. Place brisket and short ribs in a large baking dish. Pour brine over to cover completely. Cover and chill for at least 8 hours or overnight. Remove meat; rinse and set aside.
For bouquet garni, meats, and vegetables: Place first 6 ingredients in center of a triple layer of cheesecloth. Gather up edges; tie with kitchen twine to form a bundle for bouquet garni. Wrap marrow bones in cheesecloth; tie into a bundle with twine. If desired, tie oxtails with twine around circumference to keep meat from falling off bones.
Place brisket, short ribs, bouquet garni, marrow bones, oxtails, beef bones, veal bones, veal breast, 2 chopped carrots, celery, and onion in a very large heavy pot. Add water to cover meat (about 7 quarts). Bring to a boil, skimming off any scum and fat that rise to the surface. Reduce heat and simmer, skimming occasionally, until short ribs are tender, 22 1/2 hours.
Transfer short ribs to a 13x9x2 baking dish; add 4 cups broth from pot and tent with foil to keep meat warm and moist. Add sausage to pot; continue simmering until sausage is cooked through and remaining meats are tender, about 30 minutes longer. Transfer sausage, brisket, oxtails, and marrow bones to dish with short ribs.
Place a large strainer over another large pot; strain broth, discarding remaining meats, bones, and other solids in strainer. (You should have about 10 cups broth.) Return broth to a boil; add rutabagas, cabbage, potatoes, and 2 pieces of carrots. Simmer until vegetables are tender but not mushy, about 30 minutes.
For sauces and garnishes: Mix first 5 ingredients in a small bowl to make salsa verde. Season with salt and pepper; set aside. Stir crème fraîche and horseradish in another small bowl; season with salt.
Transfer vegetables to a platter. Thinly slice brisket against the grain; cut sausage into 2 pieces. Return meats to baking dish.
Season broth in pot to taste with salt and pepper; divide among bowls. (Reserve broth from meats for another use.) Serve meats and vegetables with salsa verde, horseradish crème fraîche, and both mustards in small bowls alongside. Serve with toasted country bread.