At low altitude, even if the pumps would have encountered such conditions as you describe, suction feed bypass valves in the engine feed system would have opened to feed the engines. Those are check valves and not sensitive to the conditions the pump impellers are subjected to according to your explanation. No ice crystals would precipitate out of solution at those suction feed bypass valves with the fuel quality being within specs as tested by the AAIB. A scenario whereby both the boost pumps and the suction feed bypass valves would have been blocked by ice therefore seems very remote.
Thanks for reminding me of the alternate fuel route. I agree with what you say in regard to the suction feed bypass valves.
I don't think that this (alleged, but assuming it did occur to some A/C) LP pump icing, particularly accumulation of ice in the area of the LP discharge check valve, is at all likely without some severe restriction of fuel flow rate somewhere along the normal path of fuel flow. Otherwise, such ice crystals as might form in the low-pressure region of the pump would not have time to grow beyond very small size and quantity before being swept into the higher pressure region of the discharge, which would stop their growth.
So yes, it well may be a swiss-cheese situation. It's certainly possible that ice, if any, could have been just an effect of a more primary chain of events.
OE