Should I include egg shells in my stock?
I've not heard of egg shells being used that way. I'm not sure what they would add.

The classic way of clarifying a meat stock to make it crystal clear (ie: for a consommé) is to whisk egg white (and I know at least one chef who adds crushed up egg shell to this mix) and finely ground meat into the cold stock and then gradually heat it. As the added ingredients cook they rise through the stock trapping all the bits that make it cloudy and the gunky 'raft' can be skimmed off the top.