Those readings in the vicinity of the nucleus could be the product of an electrochemical process which would be most active on the nose of the nucleus where solar wind(sheet current) is in greatest contact. Of the four comets that I am aware of that they have gotten close to there seems to be very little to no presence of water. The fact that there is this discrepancy between what we can see in the visual light range and what the sensors are reading suggest that there is something more to this. We agree that there is the presence of water, what we do not agree on is how that water is present. You insist on snow or ice, even though the images do not show any. Oh yes, it is subsurface, but even then from what I can ascertain, there was none observed by sensors or otherwise when they impacted a projectile and analyzed the ejecta. While I insist it is being produced via electrochemical process stripping atoms off the rock.
True enough about the brightness being difficult to measure and compare. Thanks for the link, I will check it out and given enough time may even link up a database on sunspots and graph the two together for comparison. A single comet clearly does not define all comets. But I do think Birkeland was on to something there.