The idea that characters aren't in battle until their abilities complete is something I've heard often (and have probably said at some point) but the more I think about it, the less I agree with it or can find any reason why it must be that way. It is inconsistent with certain characters' abilities. For example, Jezebel's ability to band to King Ahab isn't considered complete until Ahab's ability is complete (that is a long established ruling regarding such cards as Seven Wicked Spirits), however, if Jezebel is not in battle until after Ahab's ability is complete, how can Ahab ever perform his second ability?
Unless someone can give me an REG quote that says so, or a reason why it should be that way, I am inclined to say that the idea that characters aren't in battle until their SAs complete is either an archaic ruling that seemed necessary before the concepts of triggered/conditional abilities were ironed out, or it was never an official ruling to begin with, and just an explanation someone used for some purpose that kind of stuck.
That said, I would rule that Rehoboam is in the main battle during the entirety of the side battle.