Just have to talk to Bolvar in your capital city to start the questline. You can skip the Maw, but sadly, there is a bug to skipping the rest ATM and you have to do some wild, funky thing with a sub lvl 70 to get it to work.
Sadly, I think it is still per character, and like I said, something about low levels. However, I am there now on a low level character and things are fine, but I am also not skipping anything.
You cannot access the Maw or Zereth Mortis on 71-80 characters who have not already begun the SL campaign. Lower levels can begin the campaign to progress to the point that you can do so. Level 80 characters can, however, access the main 4 zones through Dornogal’s Portal to the Timeways, which allows you to get into the SL and use the warband flight points there for any of the secret finder things you might need.