Love the idea of bringing mushrooms into the mix. That's a great idea! How about using dried porcini that have been reconstituted and substituting a portion of the chicken au jus with resulting mushroom broth? Sure sounds delicious on paper at least :)
Chicken base is a paste-like bouillon used in many restaurants as a starter for sauces, soups, stews and the like. Minor's is the brand name. It can be found on line or in most restaurant supply stores.
In terms of the amount, Mike, most corporate recipes are written in this manner. I would use a quarter of all the amounts. If the full recipe yields 2.5 quarts (or 10 cups), quartering all of the amounts should yield 2.5 cups, a very reasonable amount for a typical family meal.
Both recipes call for their "au jus chicken stock". Minor's chicken base and Lawry's Au Jus are ingredients required to make this ingredient. My guess is that the chicken stock lightens up the flavor of the au jus, and the au jus deepens the flavor of the chicken stock. Does that make sense?