I am new to this forum and would like to tell you that although french (no, do not hit me on the head) I am very impressed by the great honesty and technical knowledge of all concerned. I am now retired but have flown a bit for fourty years (pilot flight engineer and pilot again...).
I do not know the 777 and will certainly not play the Sherlock Homes role...
But your remarks lead me to state that to my knowledge, center fuel pumps should not be at work on finals. After emptying the central tank (well...nearly) they will only come alive again if the tail trim tank (when it exists) sends its fuel to the center fuel tank. To my knowledge, the fuel in the tail trim tank is never checked for water, as it is "put" there by transferring fuel from other tanks. But condensation might occur and lead, flight after flight, to some quantity of water resting there.The fuel in the main tanks was checked for water by BA on the very morning of the flight.
And on modern aircrafts (Airbus at least) there are inhibition phases for the central fuel pumps which prevent them from working on finals if there is fuel in the other wing tanks (pumps ON).
As for the aircraft attitude on finals, the pitch must have increased slowly as the speed decreased, to stay on the glideslope, but not much more than in the hold at FL90...so that should not explain a sudden rush of sump water to the pumps...
For a few years I flew the Fokker 100 and we had a lot of water in the tanks, and probably due to this, some cases of fuel contamination by fungus. This was apparently due to the size and ventilation of the tanks, but we checked them every morning for water and drained it. This certainly does not seem to be the case for the 777.
So: serious operator,competent investigation, and nothing yet...Should we not focus on this "wear" of the high pressure pumps due to cavitation? And it would certainly be helpful to know if this is found on other 777s or if it is the sole case to date? An important quantity of air mixing with the fuel seems incredible on both engines fuel systems, but that might explain the fuel partial starvation on finals? Not very proud of this explanation, but that plane certainly behaved in a very baffling way...