Alright, figured I would post this as in game (at least on ps4) it's been a hot issue with a multitude of theories on it working or why it's broken.
One thing we can all agree on is that is is indeed broken. However, I found out today that it doesn't have to stay that way with some patience. No doubt most of us have sat in that room with it telling us to "defend the silvenar" while 40 people frantically run around throwing theories and yelling at those killing the enemy that spawns up top.
I have to admit I did it as well and I was wrong. They were right to kill her... well half right. I found today that it isn't an issue on whether or not to kill her, it's the quest is actually MAKING you skip (albeit not intentionally) a key part prior to defending the silvenar. To highlight this I will list in my own words in order how the quest should progress.
*talks to silvenar on road or wherever you find him
1.Save ambassadors
2.Collect ritual book
3.Talk to female ambassador
4.Collect binding (or ritual stone) stone
5.Talk to male ambassador
6.In hut, talk to each and sacrifice one
7.Deactivate (or destroy) the 4 tower crystals
8.Find where he is being held\defeat captor (hint to search fortress on hill)
9. TALK TO THE SILVENAR
10. Defend the Silvenar
11. Talk to the Silvenar
Now how I figured my fix out is after sitting on Defend the Silvenar for 4 days getting nowhere, I decided to say screw it and abandon the quest\restart it. Now, once I did that and got back to step 8, I walked to the fortress door and noticed something. As soon as I reached the door it automatically put me on defend the silvenar skipping step 9 altogether. This is the glitch, it's essentially force failing you on a key part to continue.
Without going inside I stepped away from the door, logged out and logged back in. I noticed I was back to step 8, proceeded inside and noticed only a couple others were inside with me. From here it went on without a hitch. Now I'm not sure if I just got lucky with the relog or what, but I at least wanted to post this as a method for anyone else to try. If it helps someone then I'm glad I did.
And as a side note, I can say with certainty now she is most definitely NOT supposed to go downstairs and summon anything. She should be killed up top, just have to not let the quest line pass over an important step.
5 out of 6 people like playing Russian Roulette.