There is one other possibilty which could explain fuel transferring from a "good" tank to the "bad" side with the crossfeed closed. It is extreme but I know it to have happened for sure on a B757. I may get some of the finer detail wrong and I haven't dug out the schematics but it basically boiled down to a ruptured refuelling gallery or pipe WITHIN the tank and the tank booster pumps pressurised the fuel into the ruptured gallery which as I remember delivered fuel to the centre tank. Could the same thing have happened between wing tanks here?
This would of course still require an external leak such as an engine fuel line to cause the problems of the A330 and that is looking at double failure scenarios but is it possible? Any engineers out there with access to Service Bulletins? It would have been late 80's to early 90's and involved fuel lines which were constructed with a sort of spiral or twist effect and although I used the word rupture it was more a case of the spiral or twist unwrapping itself! I know it sounds weird but could anyone else amplify on this?