I also had the same problem and while looking for these "survivors" I realized something. Before starting the dialogue with Zevlor, there's an archer named Lakrissa. She's a merchant who is listed as "neutral" and not as an ally.
But the problem is that when you start the chat with Zevlor and the fight begins, she suddenly disappears and is impossible to find. So I went to assassinate her in a corner (if you don't use turn-by-turn she'll run away, so be careful) and then I assassinated the other archer named Arka. This time, the fight against Zevlor simply started, the quest updated and, after killing Zevlor, I was able to join Minthara directly in the refugee room.
Another interesting point is that, having Wyll on my team, I didn't have to kill him, but he just left after killing Zevlor.
I hope this helps!