I'm no expert on mistranslations, but as long as Sakurai's original comment stated something about Aether returning, then it would basically mean the same thing. And considering it's a short sentence with Aether as the focus, I find it unlikely that it would have been so messed up in the translation process that it would lose its intended meaning. Though I suppose we could always just find someone who can read Japanese and have them translate the original.
Now, about the Id~Purpose matter... I have to wonder why they would choose that song specifically to have on Arena Ferox. First off, the 3DS stages only get two songs each, so I assume they have to be pretty careful about choosing which songs they want to use. Not to mention that it's highly possible that the first song would be a rendition of the main Fire Emblem theme, so that would leave one slot for a song left. So why Id~Purpose? The song itself has no connection to Arena Ferox, so it's not like it's basically required for the stage, and the song happens to be very closely tied to a certain main character in Awakening, so having the song without the character to go with it would be a little awkward. And there's plenty of other great Awakening songs they could use for the stage ("Don't Speak Her Name" comes to mind), so why would they go with Id~Purpose? I can think of two explanations for this:
1.) Sakurai just really liked the song and decided to have it play on Arena Ferox, not caring about how it's the theme song of a character who a number of people wanted in Smash.
2.) Sakurai decided to make Robin a playable character, and chose the song specifically for him/her. After all, it would be a perfect song to use for a reveal trailer.