If you have leftovers, refrigerate the pork in the sauce--it will help keep the meat moist.
Very generously season pork with coarse salt and place in a 5- to 6-quart slow cooker. In a small bowl, mix together fennel seeds, red-pepper flakes, marmalade, vinegar, rosemary, and garlic; pour over pork. Cover and cook on low until meat is pull-apart tender, about 8 hours.
Meanwhile, combine couscous, 1/2 teaspoon coarse salt, and oil in a bowl; add boiling water. Stir, cover, and let stand 10 minutes. Fluff with a fork; stir in chopped watercress.
Transfer pork to a large bowl with a slotted spoon. Strain sauce through a fine sieve, discarding solids. Skim fat from surface of sauce.
Spoon pork and sauce over couscous in serving bowls. Sprinkle flaky salt over pork, and top with watercress sprigs. Squeeze orange wedges over dish and serve immediately.