I cant comment upon the interpretation of the pictures being night or day. In my view there is no indication that it is day or night, and to be fair, there does not seem to be any indication that the texts as to whether it occurred in daylight or not.
I imagine most battles took place during the day (ignoring night raiding and skirmishes), but wasnt the 776 stuff a siege, which would have been ongoing day and night?
There is perhaps a way to ascertain the time of year of the event, and that is to compare the 14C concentrations in tree rings growing in northern and southern hemispheres, since 14C uptake would only occur during the growth seasons, which are 6 months out of phase with each other between northern and southern hemispheres. Whether measurements could be precise enough to ascertain this I dont know, but it could be theoretically possible, assuming an instantaneous uptake of 14C (which of course there may be a time lag on depending upon atmospheric mixing).
I am not sure of the similarity of a cross with aurora, but perhaps it is seen as being symbolic. As the authors of the article I linked to indicate, the red cross could also be translated as "red sign of christ", and hence could be confused as meaning cross in appearance, as opposed to a Portent delivered to the Christians.
I would suggest that the 765 entry could be depicting/describing a bolide as opposed a comet. The reason being is that it is described as a swift dagger (according to my novice translation), not as a Cometa. For example, see the entry for 761 which could be describing the 760 apparition of Comet Halley. The description of 765 does not discount a comet, but given the word comet is not used, and given there is an emphasis of speed, a fast moving atmospheric bolide could be the likely culprit. Indeed, the text suggests a notable meteor shower/storm "followed" by a quick dagger, and could indicate a notable fireball following this shower, which would be tempting to link the two, but may not necessarily be the case.
Jonny
The path to good scholarship is paved with imagined patterns. - David M Raup