1. Pull and carefully cut skin from duck legs without cutting meat; set skin aside. Trim and discard any fat from meat. Mix sugar and salt in a large bowl; add duck legs and rub with mixture. Cover duck legs and chill at least 30 minutes or up to 1 hour.
2. Meanwhile, preheat oven to 350°. Lay duck skin flat on work surface, fat side up. With a sharp knife, slice off any thick areas of fat so fat is even; discard trimmings. Stretch skin flat, fat side down, in a single layer in a rimmed baking pan. Bake until skin is crisp and deep golden, 25 to 30 minutes; occasionally turn pieces over. With tongs, transfer crisp skin (cracklings) to paper towels to drain. Reserve 3 tbsp. duck fat; save the rest for another use (see Notes). Coarsely chop cracklings; set aside.
3. Rinse duck legs well, rubbing gently to help release salt. Pat dry.
4. Pour 2 tbsp. reserved duck fat into a 12-in. frying pan over medium-high heat. Lightly brown 1/2 of duck legs, turning as needed, 5 to 8 minutes; reduce heat if pan starts to scorch. Repeat with remaining duck. Transfer duck to a shallow baking dish (2 1/2 to 3 qt.).
5. Add onion to pan and cook, stirring, until soft, 4 to 5 minutes. Add broth, tomato sauce, thyme, olives, and peppercorns. Reduce heat and simmer about 10 minutes, stirring occasionally to blend flavors. Add orange liqueur and orange zest. Pour over duck, cover dish tightly with foil, and bake until duck is very tender when pierced, about 1 1/2 hours.
6. About 15 minutes before duck is done, cook pasta in salted boiling water according to package directions. Drain pasta and return it to cooking pot.
7. Stir remaining duck fat, the bread crumbs, and cracklings in a large frying pan over medium-high heat until crumbs are lightly toasted, 2 to 3 minutes.
8. Skim and discard fat from duck sauce. Mix about 1/2 cup sauce into pasta. Arrange pasta on a large platter and top with duck legs. Spoon remaining sauce on top and sprinkle with parsley and crackling mixture.
Note: Nutrition analysis is per serving.