8 gallons in the tanks at the planned destination?
In the DA 42 fuel burn is 6G/hr per engine at 80% power.
So 8 gallons remaining over two tanks is actually (almost) required IFR reserves of 45 min at 75% power.
No matter how you look at it it's legal but fairly tight for a 500 NM flight.
If this aircaft has the long range tanks installed and filled with an additional 26 gallons the aircraft would have been 182 lbs (26 x7) overweight with the previous assumption of 175 lbs/pax + luggage.
The CG arm for the aux fuel tanks is 126 inches which is almost the same as the rear passenger CG arm which is 128.
This means that with 2 rear passengers and fuel in the aux tanks max aft CG is exceeded. It essentially means the weight of three adults in the rear seats.
Exceeding aft CG plus engine problem.................
**** the above is all purely scenario based speculation and does not imply in any way shape or form that I know or claim to know the cause of this accident ****
For all I know they had three fuel stops planned and no aux tanks installed.