'Bur Erinnerung an meine Dieenstzeit' Seems to mean 'In remembrance of my service'.
I suspect that the men in the 3 pictures at the top are (L to R) Umberto I, King of Italy/ The German Kaiser/ Emperor Franz Joseph of Austro-Hungary. 1882 is the year they signed the 'Triple Alliance'.
The scenes to each side appear to be generic military scenes rather than specific incidents.
Looking at the 3 figures in the central scene you have already pointed out their apparent oddness. Additionally, you may note that they are very white in the face, specially compared to the other heads at the top and to the sides. My suspicion is that this is because the panel was mass produced, then identifiable faces would be added later, but I am not sure about this. It might be intended that 3 individuals who were friends would be portrayed. If so then the heads were completed by a different artist, and at a later time, than the rest of the panel. Can you pick out, by close inspection with a magnifying glass, differences in production methods or ink type between the 3 heads and the rest of the panel ?
The ancestor you identify would have been too young to take part in the notable military campaigns that led to German unification which culminated in the Franco-Prussian war of 1870-71. He would, however, be old enough, at 22, to be in the military in 1882.
Unfortunately, the picture is not in focus so, even by expanding the picture it is difficult to see much.