The idea of a fall back command is nonsense. We had a long discussion about this before. It didn't happen historically in this period. Armies that go backwards are defeated ones. It's not possible to move the formations backward facing forward, unless they're being forced backwards by an enemy force.

if you want this as a feature then you're going after mafde up tactics. You might as well wish to give your army wings. That'd be useful too. Ancient armies, which this game is designed to model, were designed to go forward, especially the Romans. They were a meat grinder - you pointed them and pressed go. Good generals could do more with them, but it will always involve changing direction and formation, never dropping back in formation.

If you want to go backwards you deserve everything you get :)

Before anybody cites Cannae or Zama go and read Polybius properly.