Earlier quoted context omitted.
Is that your worst story? I read it like this: Border officer didn't want you to take a car with a wrong/likely-illegal registration across the border - instead took some time and effort to clarify the details and then allowed you to continue. In my mind that officer was doing their job well, and acted rather generously in your favour.
The registration and title were in my name because title had cleared the German DMV. He basically didn't believe his own DMV and started calling previous owners in the middle of the night.
As best I can tell, Germany hasn't had any road border controls since 2007 (when Czech Rep & Poland implemented Schengen).