I'm not sure if it replaces the default track but you're allowed to add multiple songs to an area and it will randomly cycle different ones every time you enter that part of the game.
Yeah, they are eventually going to add added stories for each character although if they are the same caliber as Lent's added story then they will all be not worth the asking price in the slightest. I believe they are also adding a whole new area to the game early next year that will feature very hard monsters but greater materials to farm. Not sure if they plan to continue the story with more DLC. They will probably just go straight to Ryza 2 for that.