ok...if the fuel was contaminated....it is MORE Likelythat a problem would have happened on takeoff, not landing...takeoff is using alot of fuel...approach, not so much.
so I don't think it was contaminated fuel
AS to shutting down the wrong engine's fuel lever...well, its possible, but I will tell you this...if one engine quit on short final, I would just land and not do the checklist for engine out...I mean, its not like you have to feather a prop on a jet!!!!!!!! IF it were a fire, that would be different of course.
The fuel system is really quite elegant and does NOT need a flight engineer. There is one failure mode which can be almost disasterous, but it would require rotten mx.
Two engines out at the same time....very unlikely....and yes the DC9 was the first jet transport certified without a flight engineer...at least in the USA.
IF this guy had 8000 plus hours in type, I really don't think it will come down as pilot error. The whole DC9 series is so elegantly simple (KISS) and so reliable...provided MX is good. And anything you do, like select crossfeed to balance tanks, an experienced pilot knows that you have to watch it and you put something like a checklist between the throttles to remind you to close the valve...provided the cable is well maintained between handle and valve.