RE: Engine temperatures.
I doubt that boiling water would work well, since the emissivity would be so much different than non-polished metal. Same for ice.
To answer the question, I would guess that the IF temp gun is more accurate than the on-board gauge. You could test it by checking some large unpainted bolt (say from a picnic table etc) that is in the shade. That should give you a ball park figure as to how accurate the gauge is, profided you know what the correct ambient temperature is.