Food Safety of "Scavenged" Chicken Stock
I use frozen trimmings and bones for stock on a regular basis and haven't ever had any issues (and I can't see why you would).  However, parts from food eaten by someone may have contamination issues unless they only removed the bones and/or skin with clean hands (which you could have done yourself before serving them).

I don't follow many rules for my "stock" either. I figure I can get vegetable flavor from vegetable stock, so I keep my meat stocks just to the meat trimmings and bones as much as possible (I do regularly and unavoidably end up with herbs and spices from rubs and marinades), let them simmer in a crockpot for days, and then reduce them. They are not clear like traditional stock, but I rarely need a clear stock--mostly I just want to extract all the good flavor for sauces, soups, and stews, and that is very successful with this method.