As to the historical part of the question, we did make a very conscious decision to keep the open enough to allow ahistorical actions. It's meant to be a light Euro-wargame crossover, not a finely-tuned simulation by any means. During playtesting some players wanted the ability to chase barbarians outside the bounds of the empire, and we decided to allow it.
And generally there were plenty of occasions when the Romans did cross the Rhine to take the fight to the Barbarians, or into Scotland, or to North African tribes. Not sure that all those raids happened during the time period here, but certainly the Romans, even when defensive, sometimes felt that was best done on the offence so it's not wildly ahistorical in any event.