In ONUS! the player that has the turn is the attacker. And he chooses which 1 vs 1 combats come first. Once a 1 vs 1 combat is over we go with the other one taken into account the previous wounds.
When playing the counterattack the defender (according to the above rules) becomes the attacker. But just for the counterattack. The rest of the turn she is the defender.
When two or more units are engaged in a melee combat they do not have to be activated. But if you want to move your unit that is engaged in a melee combat you must activate it first.
In your example, if one of the Carthaginian units wouldn´t be there then the Roman would need to activate in order to circumvent the Carthaginian one. Both units would have 6 sections in contact (this is they would roll 6 dice) and both will be flanking the other (-1/-1 defense and -1 morale)