The book provides almost no suggestions or advice for the DM - pretty much the only eventuality that's discussed is what to do if the party takes the most stupid approach and tries to absoutely positively kill every demon lord in the room.
That is not my recollection of the discussions involving the last battles.
</runs off to get book>
I find the relevant discussions on page 123-124, where it specifically talks about walking across the Grand Council Chamber and what conditions lead to battle, which are very few (such as passing the Throne "too close", and it's a 5' squeeze when you look at the map, so it's certain). As long as they don't try and engage everyone in combat at once, and aren't displaying the demon-slaying weapons openly, and don't approach the demon lords, combat that isn't going to happen.
It does mention that players can try and talk to some of the lesser beings accompanying the demon lords. This had better go with excellent bluff rolls and can't go on forever. Repeatedly questioning every group would, in my opinion, look suspicious by itself.
On page 127, column 1, it gives a more detailed account of what happens from the time the players enter the room.
One option is to enter the Grand Council Chamber and seek an empty spot to wait, none too close to any of the others. When a demon lord returns, attack Eccozt at that time. Treason and Lolth's Fury will assist Eccozt, I believe. Since I also believe that one of these two would be the one to ask the next demon lord to go to the Audience Chamber, one will not get the summons. They must then attack and defeat Gethshuq to move on to the Audience Chamber. The four must be defeated, hopefully in two fights, within thirty rounds or Lolth's envoy comes out to investigate. The others will do nothing during the battle, per the earlier description.
If the players go into the Audience chamber and Lolth does not emerge within thirty rounds, Graz'zt seeks to discover what occurred and proceeds to the Audience Chamber. He will be using stealth on the way in. If Lolth is victorious, he gets out and acts innocent. If Lolth has lost, he gets out, announces that Lolth's aspects have been slain by the mortals, that they all did not swear the Pact quickly enough (for future impact), and then he leaves having recognized the pointlessness of the current activity. He will seek revenge on the mortals in question later, just for evil's sake. It might be possible that he will persuade some others to attack the PCs, but that's up to you.
One problem with a general teeter-totter setup like this one is that there are many opportunities for mistakes that can lead to the party's death, and given your typical party of players, they are relentlessly likely to come up. One player suddenly decides that running up to a demon lord, throwing himself at its feet, and swearing eternal servitude, is actually a good idea, for example (I kid you not). Another decides to engage all evil on the spot, just because she's a paladin.
If the players are going to be silly, they're going to die. It's hard to stop that.
You need to take charge of this encounter, think through common possible player actions, good or bad from your viewpoint, and be ready with your responses ahead of time.
Example: Player Action: The party persists in trying to talk to Pale Night after being subject to her first insanity attack. Pale Night Does: X, Y, Z.
Example Player Action: One player marches directly between Baphomet and Yeenogu, and starts taunting them both into attacking each other. Their Response: They attack the player and then, in a rage, each other. The party supports the player, or not. The player (and/or party) survives, or not. The other demons lords look on, wondering if they should do something. If the two demon lords dig into each other for more than a couple of rounds, Treason runs to get Lolth's envoy. If one of the demon lords dies, Pale Night departs by ordinary means (she only uses her teleport ability if necessary).
The aftermath of defeating Lolth's aspects is that Lolth herself (presumably at some very high CR) shows up to chase the PCs out of the Demonweb.
Having excellent healing support along is going to be critical to making it out.
Exactly how the PCs are supposed to figure out all the specific tactics? Maybe they'll have to talk to the demons after all (perhaps one is trying to throw the Pact and gives real advice to help the party). Maybe they can get a divine prophecy to help them out.
In the Nest of Webs
When Princes Pass
The Warrior in Jet
Shall be the Enemy
Yes, it's overly obvious. If it isn't, it won't help.
