Hmm..Sloane Ranger good point about when the Alliance started. So, perhaps, Sloane still felt that Jack was disillusioned with the CIA and disillusioned with alot of things in general. He was pretty messed up for awhile after his imprisonment. So, when Sloane joined the Alliance, he probably thought that Jack would have no problem joining, I don't know. #3 seems the most likely. Jack has said that when he broke into Langely archives to find out about Irina investigation that was the first time he broke CIA's trust. I don't think that the CIA would have welcomed him back so easily if he had left them at first to join the Alliance. He seemed to be a pretty high ranking officer and respected at the CIA.