I totally get why you enjoyed making this recipe. The ham on the bone cooking method you described sounds pretty straightforward, so I'll definitely give it a try.
The gray residue after cleaning could be built-up grease or just signs of use. Over years of use, cast iron can develop a variety of appearances, but this is normal and not related to cleaning mistakes.
If you have an oven, you can make mac and cheese in it. Just cook the macaroni, then drain it and mix it with cheese and other ingredients in a baking dish.