UPDATED: This has been resolved. When I migrated the Sopwith Triplane folder into a newer install from a previous one, an older name in the Nations17.ini file apparently caused a conflict that resulted in the described condition (not sure why FE2 would only limit Single Mission aircraft to one and then let everything else proceed normally – anyone have an idea on this?). Circling back and recognizing which NationsXX.ini variant was controlling the “country” names in each install was the first step, and then further checking and correcting the various .ini and Texture sub-files for each aircraft finally straightened everything out. A time-consuming job but everything seems to run OK for the present!
Silberpfeil's post in Sopwith Triplane question was marked as the answer