I would say that they either they have move out after the battle.
I think you are saying they'd have to leave after the battle, which brings up questions like where do they go? Can you end up with units in different places than they started?
You said the thing you like about the combat was the simple elegance of it. I think it would be more elegant not to be moving units all the time, so it'd probably be most clear/elegant to just pick the "Contested" space or tile and then all the units touching it are automatically on offense and all the opposing units touching it are automatically on defense.
I would say that they either they have move out after the battle.