I don't know what motivated Clinton to actively campaign for Lieberman, but I think Tom Schaller's engaging in a bit of wishful thinking to imagine he did it in exchange for Lieberman promising not to run on the Lieberman for Lieberman ticket if he loses the primary. I suppose it's possible Lieberman made the promise, but he already made and seemingly broke that promise to other senators. If he pulled petitions to run and is now changing his mind then he made a tremendous campaign blunder.
It would have made sense for Joe to float rumors about running as an independent for a couple of weeks and then instead of announcing that he was pulling petitions give a stirring tribute to the amazing wonderfulness of the Democratic party. He could've said that he's had to ignore his advisors who are begging him to make the jump because he just can't imagine running as anything but a Democrat, blah blah blah. It would've put a stop to the basic Lamont campaign narrative/momentum which had for weeks been built around Joe jumping ship.
But Joe couldn't do that, and now I imagine he's committed to his course.