Can my hero complete all faction questlines (i.e. The Travelers, Warsorn etc) in the same playthrough or is it limited to only 1?
You can complete all faction questlines on 1 playthrough/character/save, or whatever you want to call it.
Yes, you can join and get missions from all different factions in one playthrough. I have finished the House of Ballads quests already and I'm making my way through the Travelers, Warsworn, and Scholia Arcana missions all at the same time with one character.