We don't really do them on a big enough scale to warrant getting a mincer, but that's an option for tough meat I've not come across before. We go through a lot of lamb mince, as well.
At this time of year we use the pressure cooker for stock, so they don't need more than a few hours. In winter we can put the cast iron pot on the woodburner for the day.
Do you use vinegar in the stock to leach out the minerals, etc? If you used the cast iron, the vinegar probably would not be good for it.
I haven't heard of that before, so no. By the time I'm done the bones are pretty much crumbling, so I figured I'd got the best from them. Yes, probably not a good idea with the cast iron.