When was it ever said that it was only 5-10% of Igneel's power? Sure, Igneel wasn't at full power, but 5-10%? There's no evidence of that. Also, there's no guarantee that Metallicana or Grandeeney were on the level of Igneel to begin with. What Igneel did was more impressive than what the other dragons did. Igneel fought Acnologia for 20 minutes, and knocked him down once, and then took an arm. There was a real threat of combat. The faces were just stationary targets.
Everything you're saying about Acnologia's parent dragon largely speculation. We know for a fact that dragon slayers must have killed their parent dragons, or at least the parent dragons of other dragon slayers. Even if Acnologia didn't kill him, whats to say that one of the dragon slayers from 400 years ago didn't do the job.
Dragons do use dragon slaying magic. It was stated that "There is ordinarily no way for a human to learn dragon slayer magic; It is a skill restricted to the dragons themselves".
Link here. This means that dragon slayer magic was a skill that the dragons used. It makes sense too. Zilconis even said that dragons can use magic, so there's no reason a dragon can't use dragon slayer magic. The dragons must have invented the magic, and know it to such an extent that they can teach it. So why wouldn't they be able to use it? Dragon Slayers use the same power as their dragons. That's why Natsu's fire is recognized as Igneel's, and Wendy's sky magic was recognized as Grandeeney's.
In practice, you can obtain dragon slayer magic by lacryma and/or taking it from someone by eating it. Practice beats theory. My point is that if Etherion contains all types of elements within it and Acnologia has the elements of all the dragons he's stolen souls from, then Acnologia's power should resemble that of Etherion. When you look it it, the colors of Acnologia's roar does kind of look like Etherion. I mean, what element do you think Acnologia has then?