Yeah, you've discovered the issue: I'm pretty attached to the idea that Natia is in the party when Sera meets up with them in the Deep Roads. And I kind of want them to see the surface at the same time, because it sets up some differences in their personalities. One thing that might work, maybe, is having Natia hanging out at a camp in the Deep Roads with some of the Wardens while Duncan deals with the Aeducans? I'd have to explain why he's willing to wait a few days to leave for the surface, though, when he's always in such a hurry to get to Ostagar.