I follow you
At 8,000 feet,the water boils at lower temp.(lower pressure). This system is entropically related, all variables change to accomodate a conservation of energy in the system.
My Bottom Line is that Fuel, Emulsion, whatever is in the Line in the aircraft can Freeze simply by experiencing a sufficient drop in Pressure.
If you can grant me that, the rest isn't germane. Are you saying that Water always freezes at 0 degrees C.? The soda bottle can be below 0 Degrees with no ice, open the cap, instant ice, same low temp. The freezing point has migrated, but not because of Temperature.