I tried to do this mission several times and never succeeded. I always got stuck on the part of the speech in the great hall, after speaking with magnifica falorah. To my surprise, after returning yesterday from another mmorpg, I found that I was able to finish the quest normally on my main character after several years of trying since the launch of the thieves guild. The mere fact of having achieved this made me think about going back and completing all the quests, as I initially intended, on my main character. It also reminded me how good elder scrolls online quests are. Unfortunately, however, the ground finder is broken, but that's the least of it for me. It's good to be back.