Hi,
I think it harder than that... I have seen old engravings on regular silverwear where the piece istelf is patina covered... but the inside of the engravings silver... and polised silver pieces were the inside of the engravings were filled with dirt...
Also, it is possible to have a late award, barely worn, with an engraving, then kept in a drawer... or someone who got a cross in 1914, wore it throughout the war, then had it engraved in 1920, then wore it further on other uniforms.... or an engraved cross given as a present and never worn....
There are probably as many combinations and posibilities as there were crosses awarded.
As far as people doing the engraving.... most old crafts still have a hard core on enthousiats who practice just to keep it alive... we are all digital, but plenty of people who are still hardcore and develope and print photos in the basement... or shave with a staright razor... or dont buy readymade curry powder but mix it themselves.... and, as such, there are still hobby engraves out there, maybe they are not as good as the topline engravers of back then, but they are surely as good as the "average" engraver of back then....