1) No you don't have to be a German national.
2)Flights must include Frankfurt Hahn or Cologne. So yes they can be from any city but can't be (for instance) London Stansted - Brussels Charleroi.
Hope that helps and enjoy your trip, if you book.
Goodbye SR-LX MD-11 / 6th of March 1991 to the 31st of October 2004