In light of only 300 kg imbalance between left and right main tank, it is also ruled out that one of the crossfeed valves could have been (partially) open undetected.
I'm not sure about that Green-dot, if crossfeed is open but fuel pumps are not switched off on one side or the other, each wing tank will still feed its own side engine unless fuel pressure is somehow stronger on one side.
If I'm correct one crossfeed will automatically open under a certain type of
electrical failure ... !?
... You mentioned that, I mentioned that, it would be nice to know exactly when the APU flap started to move ?