Its for the whole period that the splatfest t-shirt is available so starting as soon as you choose your team just over a week before the fest itself. You have to hand the shirt back the second the fest ends.
That way costs loads more money and you'll need to exit out of the lobby every few matches to see Merch for the wipe. Better to fully level each time and use a drink from Crusty Sean to get the 1.5x gear exp plus greater chance of chunks you want.
In ranked you get a bigger win bonus based on your rank but for all game types points in game are irrespective of level or rank
In turf war & splatfest a team that does not ink its 'home areas' will almost always lose.
In ranked modes: splatzones, rainmaker, tower control there is no need to ink your 'home areas', the objective is much more important. In rainmaker and tower control having inked your 'home areas' in particular the bits near the goal can help on defence but if you focus on it too much you will likely lose.