You know, I always thought that Obi-Wan's revelation about the Death Star not being a moon was because he could sense something through the Force. I'd imagine that a big hunk of metal infested with thousands of living beings would have a bit of a different feeling in the Force than a small moon would. I think that Bail sent Leia to get Obi-Wan because things were not going good for the Rebelion. Ol' Bail wanted the last of the Jedi to come to their aid and maybe help them score a victory, which he did. Although, I do like the idea that maybe he was sending Leia to be trained. I find that very intriguing.