Am I right that its perfectly legitimate to not assign a leader to a defensive battle and then (assuming you win) to move the forces used in that defence on a later movein the same turn? It seems to simultaneously comply with the letter of the rules while violating their spirit.
I don't think it violates the spirit of the rules at all. If you could guarantee freezing a force by attacking it, even with almost nothing, it would be nearly impossible to reach the Rebel Base in force. I think it's clearly intended to work exactly as it does.