1. Set slow cooker to high to preheat. Drain beans and transfer to large Dutch oven. Add 8 cups water, baking soda, and bay leaves. Bring to boil over medium-high heat, then use wide spoon to skim off any foam that rises to top. Boil 15 minutes. Drain beans and transfer to slow cooker insert, discarding bay leaves.
2. While beans are cooking in Dutch oven, score fatty side of salt pork and cut into 2 pieces. Place scored side down in medium nonstick skillet over medium heat and cook until fat is rendered, 8 to 10 minutes. Turn salt pork over, add onion, and cook until lightly browned, about 5 minutes. Transfer to slow cooker insert. Stir 1/4 cup molasses, brown sugar, and boiling water into slow cooker insert.
3. Cover beans with piece of aluminum foil, then cover slow cooker with lid. Set slow cooker to low and cook until beans are tender and creamy, 10 to 12 hours. (Alternatively, cook on high for 5 to 6 hours.).
4. Turn off slow cooker and remove lid and aluminum foil. Stir in remaining 2 tablespoons molasses, mustard, and vinegar and season with salt and pepper to taste. Cover slow cooker with lid and let beans sit until sauce has slightly thickened, 15 to 20 minutes. Serve.