Yeah, in truth, "power metal" is probably the single most haphazardly bandied-about genre marking, historically speaking, in the history of metal. Looking through old magazines, I've seen the term connected with Metallica, Venom, Bathory, Celtic Frost, Mercyful Fate, Metal Church, etc. There was no "power metal" genre at the time, so it's only in retrospect that we have distinguished which bands in the 80s were power metal and which were heavy or speed or thrash metal. I would imagine it wasn't until the 90s that what the average person would identify with the term came to prominence. Well, I don't think most of the bands associated with the term even existed in the 90s, so I guess that makes sense. If you're looking for "the first instance in which the genre term was actually used properly", I don't think you're going to find it.