I like the idea that they get screwed over at the end. However, you don't want them to feel like negotiation is never a viable option. Sometimes it should work, I think.
My group will be hitting Bhal Hammatugn next session.
My party knew a fair bit about Zenith, and used negotiation to get across on the boat. However, they were plenty nervous, and had itchy trigger fingers. When they got inside the entrance, and more guards showed up - who weren't hostile, but were belligerant - the party attacked.
However, if they had continued to negotiate, I would have had the Kuo-Toa take them all the way to Zenith. Then they would have had a fight with Zenith and just a few Kuo-Toa guards (to make it interesting). The high priest would have remained behind.
Aushanna would have appeared, at the statue, after the party was in with Zenith. Not knowing what was going on, she would have taken a few rounds to find out, only teleporting in to Zenith's room just in time to keep the fight "interesting"...
I don't like stifling diplomacy. It seemed to me that the Kuo-Toa revere Zenith. If the party could come up with enough bits to string the K-T along, and make them think that they were there to help Zenith out, then the K-T would take them to Zenith in the hopes of getting more religious insight from him.
Possibly the party could even have diplomacied Aushanna and Zenith...
Anyway, that's not how it happened for us...
Here is what I came up with, but I'd like to hear your suggestions.
That looks pretty good. Like others in this thread, I do believe that diplomacy should have some benefits. (My players - again, like others here - had itchy trigger fingers and fought. They were beaten many times, badly.
The only thing I'd look carefully at is:
They emerge to find dozens of kuo toa awaiting them. If they look ragged, the kuo toa attack them to provide great sacrifices to the Sea Mother. I'll position the kuo toa so the players have an avenue of escape.
This would (read: did) wipe out my PCs... for some reason, they really struggled with Dhorlot (but beat Gottrod handily). Only you know your players, but this is the one place where it's good to analyze possible results...
My group got to Bhal-Manatugn last night and chose to go the negotiation route. The drow rogue/sorcerer spoke undercommon and was able to communicate with the KT. She had also cast comprehend languages as they were approaching the ferry and thus was able to understand all of the KT speech. The two barbarians were itching for a fight and the paladin, who stated she was going to run detect evil continuously, didn't care for simply talking either.
After some outstanding diplomacy rolls the group got an audience with Zenith. I made him a gestalt character, like the party, so he was a fighter/paladin and dwarven defender/blackguard. I played him like a belligerent a&!!$%# the whole time who kept insulting everyone and pushing them around. The party nearly came to blows with him several times but managed to keep themselves in check.
After even more outstanding diplomacy rolls the evil rogue was able to play to Zenith's inner bastard and convince him to return if for no other reason than to kill his father. So he shouted some orders to the KT to ready themselves for war as they would be marching on the surface. He would send message within a week. The party understood everything he said.
They then walked out as the KT mustered to march. On the way back, by way of the kopru ruins, the party realized that Zenith was leaving trail markers and they were being followed by a squad of KT.
So they handed Zenith off to "Celeste" and "Davked". I am running Celeste as a polymorphed Thifirane so when the party warned her that Zenith was evil and planned to kill Davked she pretended to care and then teleported them all away.
So the party left a band of KT in the underdark preparing to march on the surface. The party wants to raise a war band and return to take out Bhal-Hamatugn before the KT can march. I'm thinking I might let them do that before I launch them into the next chapter.