the island was not conveniently placed..you have to go a long way to reach the sky islands (as gan fall explained back in skypiea)..so it's not something you stumble upon while traveling from one place to the other..is a place when you go on purpose,and also knowing it's there,something impossible to do from the sea level
i think i't more of the alliance getting on his way really...knowing kid he probably attacked the guy because he was on his base...
there is only one place where the highly dangerous guys are taken,and it's impel down,so jack knew exatly where the convoy with doflamingo was going.
when the convoy leaves doesn't really matter since you can,you know,wait for them to pass.
The route is only one,the one line that starts from dressrosa and ends at fishman island..when you know those two locations on a map and you make a line between point a and point b you have your route..as long as you are navigating in the grand line,you are following a logpose (in this case an eternal pose),and what it does is ALWAYS point in that direction,so they have no reason to not follow that line.
It would be diferent if they didn't had an eternal pose and had to travel between the islands,but I can't believe an high security convoy with an ex shichibukai as the prisoner will not do a direct route,let alone do stops in the various islands with different registrations time to wait.
i'm no japanese expert,but would even more formality mean distance instead of familiarity?familiarity is when you use an informal style..if he is being overly formal he is distancing himself from kaidou even more than usual...