1) No, there is no difference. In fact, when it comes to combat EB has no concept of "damage". All soldiers in EB have a single hitpoint (the exception are elephants, Gaesatae, Tindanotae, officers and generals). If they lose it, they die. Chances of death are determined by two factors: the discrepancy between the attacker's attack factor and the defender's defence bonus, and the lethality of the attacker's weapon. The former determines whether a strike was successful and the latter is the chance that the successful strike translates into a kill (if it doesn't, the defender is merely knocked down). Also important is the speed of the weapon and whether it is armour piercing (the latter applies to axes, most curved swords and long cavalry lances). Armour piercing weapons ignore 50% of the defender's armour value.

2) Yes. The soldiers are packed too tightly to defend themselves effectively.

3) The Romans can recruit Brihentin in level 5 regional barracks in several Gallic provinces. Consult the recruitment viewer (it should be in your EB folder, the one on the EB website is outdated). They can recruit Brihentinregardless of whether the Gauls have achieved the correct reforms. They are the best cavalry in the west, but phale compared to the eastern heavies. However, you are mostly correct: the Romans are supposed to do with infantry. I wouldn't call them sub-par, by the way. What battle difficulty are you playing at?