Chicken stock quantity to substitute for 1 stock cube (knorr)?
Per Knorr, you would dissolve one cube in 2 cups of water (500 ml), so you would use 2 cups of homemade stock and deduct 2 cups of liquid from the recipe.

Note that making stock from a single chicken carcass and 5 liters of water seems like a lot of water; you may wish to reduce the resulting stock until you only have a liter or two.

The stock ingredients have pretty much given their all into the stock.  All of the flavors and gelatin should be dissolved in the liquid, and the remaining solid mass is going to be pretty underwhelming.  It is probably best to discard.