I had the same feelings. I discussed this with the private seller (who happened to be Official Audi Dealer salesman), and he explained to me that whoever has the two green documents (Fahrzeugbrief and Fahrzeugschein) is the owner... So with two green documents and a 2-way signed Kaufvertrag, you can always 'prove' that you're the owner. I have the appointment at RDW at November 18, so time will tell if I'm right
JeroenSC
(ex SC Targa, now 996C2C)