There is very little room to attach a gauge to the end of the fuel rail, much less to be able to see the gauge while you are driving and the engine is under load.
It is possible that the fuel tank was on the low side and you were sucking air from the tank, depending on the incline.
No, there is no definitive way to "test" the throughput on a filter, it either works or gets so gunked up that more pressure is needed to push fuel through. That scenerio will cause the fuel pump to over heat and eventually fail. The service manual has the filter replaced at 90K mile intervals.

The higher the altitude, the leaner the mix because the air gets a little thin. Perhaps the ECU misread the altitude sensor readings? I never had problems going through CO mountain passes with my 850 or V70. But then, I do like to down shift a lot :)
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My name is Klaus and I am a Volvoholic.