Somebody mentioned electrical disruption further back in the thread, and I feel it may merit some thought here.
Question - are the tank booster pumps normally in operation in this configuration all fed from the same bus? (not a 777 operator, BTW - I'm assuming here that they're the standard 3-phase type commonly employed).
Reason being, I have come across an odd mode of electrical failure on a previous type, whereby a single phase on a single bus tripping off creates all sorts of oddities - the cause of which are not immediately obvious, since a motor with a dead phase acts as a generator on that phase (still being driven by the 2 good phases) preventing any indications of electrical failure, but sending oddly-shaped wigglyamps to other components.
Might be a worthless tangent, then again, might not be. Just something else to throw into the collective headscratching!