First off, outstanding work on this. I can't believe how much life you've breathed into this old game. Really amazing!
I checked the forum and the FAQs to see if anyone else was having this problem but couldn't find anything quite like it:
Proceeding through the main quest, I reached the Fourth Trial stage (the Hortator parts). Everything seemed to go smoothly, more or less, with the Telvanni and Hlaluu Hortator quests, and I achieved both. However, when I went to Athyn Sarethi, the appropriate dialogue option ("Tell him your story and ask to be confirmed as Hortator, etc....") doesn't show up. The only option is "Ask how a hortator is chosen." No amount of charming or disposition raising seems to have an effect on this issue (besides which, they're all at disposition 100 to me).
A note on my character, possibly important: My character is a high-ranking member of the Telvanni. At some point as I was achieving that rank, I received a quest from Telvanni Master Aryon asking me to convince the Redoran Councillors to vote to end the Mage's Guild Monopoly. My first obstacle to that quest was the fact that every Redoran councillor was at "0" disposition towards me, and no amount of charm spells, bribery or speechcraft seemed to be able to raise that disposition. So, I opted to join House Redoran, and to my surprise, they let me do so (as I recall, this wasn't supposed to be possible in the original, strictly speaking). Thus, when I began the Redoran Hortator quest, I was a Spellwright in House Telvanni and a Hireling (I believe that's the lowest rank) in House Redoran. Maybe this has something to do with it?