Something's missing
So the cold fuel transits the system, waits with liquid water in its composition so that it can freeze after entering the pylon and the enhanced heat of the HE? I think that isn't likely. Again, at the temps in the tanks, there would be no liquid water in the fuel. Something Capt. Cargill suggested covers more ground; an upstream "snowstorm" that caused cavitation at the engines. The "snow" (small granular water ice) would have formed as early as in the trucks (A/P tankage) prior to uplift.
Another problem with the Boeing memo as supplied. The assumption is the cavitation was caused by ice blocking proper flow to the HP pumps, if the blockage occurred at the HE does Boeing suggest the Fuel was flowing fine until the ice formed at the exchanger? I suppose the tests are ongoing relative to the FOHE and Boeings fully mocked up system. From the initial response of both engines one could assume that the exchangers were collecting a suspended (preformed) "precipitate" or subjected to the necking down of the exit aperture as ice quickly formed ahead of the exit.
It would be folly for Boeing to target only the RR if it suspected at all that the other engine types were in the least susceptible to the anomaly.
It would be likewise almost unthinkable for Boeing to allow a vulnerability to problems with the fuel to go unreported. This would appear to exonerate the fuel by default, the GE and Pratts as well. Let's not forget that rollbacks occurred well into the long flight, lending some suspicion to a greater concentration of particle ice with time aloft. Also, that on Delta, the rollback appears to have occurred without a demand for increased thrust, as on 038.
AF