For the rub: Mix together the salt, chili powder, paprika, pepper, brown sugar, coriander, onion powder, cayenne and cumin in a small bowl.
For the barbecue sauce: Mix together the cider and 3 tablespoons water in a small bowl. Heat the oil in a saucepan over medium heat. Add the onions and garlic and cook. As the color develops, deglaze the pan with the cider mixture to build flavor until caramelized and dark, about 30 minutes. Add the ketchup, molasses, dark brown sugar, allspice and cayenne and bring to a boil, then reduce to a simmer. Simmer for 30 minutes.
For the pork ribs: Preheat the oven to 280 degrees F. Coat the pork ribs with half the dry rub and let sit for 4 hours in the refrigerator. In a hot pan, add the oil and sear the pork rack. Place the seared ribs in a baking dish, put the butter on top of the ribs, add 1/2 cup of water and cover with foil. Cook the ribs until tender, about 3 hours. Cover the ribs with 1/2 cup of the barbecue sauce. Increase the oven to 350 degrees F and bake an additional 15 minutes.
For the slaw: Toss the cabbage with the vinegar and sugar in a large bowl and let sit for 20 minutes. The slaw should still have crunch to it. Season with the salt. Serve the ribs with the slaw.