Robin, Yes you’re right about the nucleus diameter (I was thinking it was around the same as Rosetta’s actual target, 67P, but it is quite a bit smaller. Your calculation for the bare nucleus mag is pretty good. JPL Horizons gives a nuclear mag on Dec 9 of 12.85. I get mags of 11.91V, 11.30R in an 11 arcsec diameter aperture and the R profile is definitely broader than G close in (R is mainly dust, G is mainly gas). I think you are seeing the inner dense dust cloud around the nucleus which is probably around 1.5 mags brighter than the nucleus on its own.
This plot was obtained using a widefield system with a pixel size of 2.77 arcsec.