It will definitely be a linear questline, at least at first - and will almost definitely start in the Intercessory, the room at Dragonhead where we left off the 2.55 story. FFXI showed that expansion questlines are always linear and that everyone follows it in the same way. The starting stories are 'split' because we have a choice whereabouts we start our adventure. This is pretty much just a continuation, so no choice where you begin.

Really, congestion at the start is only to be expected, it's the nature of the beast (FFXI always suffered from that for the first few days to a week after an expansion pack launched) - if it's that much of a problem just wait a week or two before starting it (you'd have plenty to occupy you enough as it is getting all your existing Jobs to the new level cap of 60 after all).