"Go suck yer blade!"
I just had my first play through with 3 of my friends. We liked the game over all but a few questions were raised during play.
1. Establishing Naval Routes; This is the biggest issue we had. Does the active player have to reestablish a naval route to travel through a sea area until the Venice token is earned or is a previous influence enough? E.g. Golfo di Venetia and Mare Adriatico are in Venice control. There are two influence tokens on Mare Libico (3) one Red and one Blue. The Red player then wants to set up one influence on Zante. Does Red need to use 2 actions to place in Mare Libico and Zante or just one using the previous influence in the sea and go directly to Zante? At first it would seem to be the latter but then why are there sea tiles w/ (5) or (4) and the note in the rule book about only 1 influence in the same sea area per turn seem unnecessary.
I strongly suggest to download the official FAQ: http://www.stratelibri.it/wp/?page_id=487.
Anyway, the answer is NO, Red doesn't need to use 2 actions (=place 2 tokens) in the SAME TURN: he does if he wants to establish a route through the same Sea in SUBSEQUENT turns (until the sum of all player tokens on the sea zone reaches the "Venice Control Token" level). This explains your question about sea zones numbered 4/5. With 3 players, it's impossibile to reach 4/5 in the same turn. Someone has to place more than one token of his color, and this will require another turn.
2. Peace Treaty; This is sort of a continuation of the above rule, if a sea route needs to be reestablished, can the new influence from the peace treaty card be used as a new connection that turn?
Yes. Question answered in the FAQ, btw
3. Mercenaries; Can this be used on any minor colony or must a naval connection be present? (Same question for the Marriage card.)
I can't remember the specific cards and I don't have the game here, but as a general rule, if the cards say "anywhere on the map", it means anywhere, no matter if the colonies are connected to Venice or not.