Will how he returns be immediately revealed or something that plays out over the story?
We will learn what happened to him between the time he "died" in Runaways, and his "resurrection" in Avengers Undercover. And to give everyone a hint, he was studying in hell, learning from the worst of humanity.
Will any other members of the Runaways make an appearance?
Maybe. It makes sense, but the Runaways don't work into any of the immediate stories I'm planning.
With the way I have things outlined, the earliest we'd see any of the Runaways is around issues 20. This is about Alex, and how much he has changed so much. At some point, Nico will have to show up, because they have some unresolved business, and she makes the most sense if anyone of the Runaways show up before issue 20. But when there finally is a reunion between Alex and the Runaways, it will be ugly.