@lilypoppy yes, this is quite on point! I'd just move the Titthi and Belli colonies closer to the Romans than to the player (while keeping them on the edge of the map), but for now I don't know where they were standing by really (their territories of origin were quite far away, and I don't think those particular forces came from there, but I have to check if this is better known).
Is the CC meant to be Tribola specifically?: yes.
Is the fight against Vetilius at the CC a real battle, or a scripted event on arrival?: a real battle would be enough as long as they are aggressive and move towards the CC, while being possible to distract them with skirmisher cavalry.
Fishes have the specificity (unlike berries) to stop regenerating if they have any gatherer on them.
So it doesn't matter much if you have 1 or 4 boats assigned they will not regenerate much anyway.