A multitude a little problems came up here, firstly when Nabina Maesa is asked about the hortator and the quest starts, the first thing that happens is a message to say that I am already Telvanni Hortator, so I speak to Aryon but he says speak to the other leaders of the Telvanni so kill Gothren, convince Dratha, Neloth, Therana, and go back to Aryon who tells me to get the support of all but Gothren who I will have to kill. Both Hlaalu and Redoran hortator are complete and because I got the message I went straight to Danso Indules who said she couldn't speak to me until I was hortator so I started the Telvanni hortator but this doesn't load, it seems to be in the completed quests so there seems to be a circular problem in that no matter what happens, the telvanni hortator quest can't be completed.
Other issue is the Ald'ruhn manor district; the door to it is there but nothing else, go through the door and I am in the manor district, come out of there, there is no giant mushroom or whatever it is.
Just to let you know, I am running v66
Were you ever able to figure this out? I just got to this point and like you I'm finding myself hit with a message saying that I am the last surviving councilor of House Telvanni, completing the quest right after getting it. I haven't gone any further.