I have not so much experience with EY Guest, but on their website they promote the "OpenSeat" compared to the "GuestSeat".
http://www.etihadguest.com/en/programme/features/

Am I correct, that the "GuestSeat" is the standard award seat, which can be limited in quantity, but there is at least 1 on every flight (as mentioned on their website) and the "OpenSeat" is every seat available on the plane, even if award seats are sold out. How many miles would the "OpenSeat" set me back on f.e. DUS-AUH?

They also promote miles + cash, which could be an option for OP, should the miles be not sufficient for the return flight.