Immo a unit with defensive orders, must defend, not attack!
This not mean they will NEVER advance. if infantry against bowmen alone could advance. Or inf/cav after some time under arrow fire, could loss it and charge, or with an enormous numeric superiority, etc... The difference is between -they could advance- and -they must advance-.
The thing about two armies, both defensive, shooting some arrow's volleys and ending a battle without any real combat... well, I don't see any problem! The battles have a maximum duration, I think... so the battle would end in a tie and all happy!