There are no cheats that let you add items to your inventory that i know of. As for the key i don't know if the problems fix if you load a previous save, you might need to restart the game.
Or unistall/reinstall the game and apply the patch, loading a savegame might work that way.
With Ancient Rusted key you can access a small area down to the deep halls, but nothing special so you might as well continue forth.
