The declination is not only difficult for those whose mother tongue is not German, but also for those ones who actually speak German as a mother tongue. For instance the Berlin dialect includes that the direct (Accusative) and the indirect object (Dative) are mixed. Still they can be understood. Another issue is the Genitive. As you have seen some forms are already not in use anymore and actually the use of the Genitive is lessening constantly. Your question might be how to describe then whose things we are talking about. Well, quite easily and like most of our European neighbours we help ourselves with a preposition von.

Das ist das Auto von jemandem, den ich kenne. = This is the car of somebody I know. Das ist das Auto von niemandem, den ich kenne. = This is the car of nobody I know.