I was able to finish this quest this time following your advice. Thank you for posting this.If anyone else is still having trouble with this: what you need to do is abandon the quest. Be patient through every step. Sometimes you have to prompt Narsis to move forward, sometimes you just need to wait for him.
ONCE YOU KILL THE MATRIARCH: do not just run into the next room, even though you can. When Narsis comes out of his cowering stupor (you may have to walk up to the door WITHOUT going in to trigger this), he'll walk up the steps, triggering the next quest completion marker, follow him into the room and finish out the conversation, you will be able to complete the quest at that point.