So, here we are 7 years later, when I returned to binge Dragon Age. And found once more invisible walls in Sundermount, and trying to Google a solution yielded naught. I once more ventured upon this thread and found it still lay unanswered.
But behold! A solution has been found!
I noticed that the issue occurs potentially in two places, either after the conversation with Flemeth in act 1, or after completing A New Path in act 3. I discovered my solution at the latter.
Firstly, enable the console. For help doing that, go here.
You won't be able to see what you're typing. Nonetheless, press the tilde key and type in "runscript zz_dae_debug". A conversation will appear with menu options.
Select Area Jump. The dialog that comes up will tell you what act you're in. Remember this and be sure to select the same act so you don't screw up story variables! Next, just select a map to teleport out to. Because I was doing this at the end of act 3 and wasn't worried about murdering an entire group of elves, I just ported to Darktown (Day) to do my next quest. For Act 1, however, you can port to any other map and then come back to do any dialogs you have left in the Dalish camp. I'm not sure if porting to the same map would take you to the area transition for Sundermount or do nothing. I didn't try it. But this works so long as you've finished your quests on the mountain.
I know this probably won't help anyone who was on this thread. I'm sure everyone has moved on nonetheless I'm posting this in the hope that it helps someone in the future. Enjoy!
