Notepad, there's a lot to chew on in your post.
You can easily find Priest's stamp on every sub genre of metal. If people want to call Painkiller their "Power Metal" album, I'm not going to argue it, even though I do not agree, and when it came out, the thought had never crossed my mind. For the few years preceding Painkiller, they were listening to how soft they had become. Plus you had the emergence of all these bands that Priest influenced going right by them. A band like Metallica is the perfect example. Priest really had to up their "Metal" on Painkiller. It's written that way and it feels that way. It's a similar thing to when Alice Cooper released Raise Your Fist And Yell.
Speed Metal was a natural progression from the NWOBHM. In my opinion, Speed Metal splintered into two, the American style thrash built on aggression and riffs, and the European Power Metal, which certainly had the speed, but placed importance on sweeping melodic passages. Helloween's Keeper Pt .1 was really the first time where I thought, jeesh, we really have something new here, as it basically laid the blueprint for what Power Metal would become.
I'm on record as saying that Helloween's are one of the most important and underrated bands of the last quarter century. Their influence on today's metal, particularly Power Metal is monumental.
And Maiden had no album that was really Power Metal. They influenced the genre in a different way, and it was really their use of memorable and inspiring choruses.