There's been talk about streamlining the adventure path by allowing PCs to begin Mother of Flies immediately after they emerge from Delvehaven. Has anyone actually done this?
I think that the Infernal Syndrome looks like a great deal of fun and I'm sorry to lose it, but I'm considering taking this advice from people who have finished running the AP.
What are the key plot elements of chapter four? Here's what I've got:
* An enormous explosion destroys a heritage building, showing how bad things have become.
* The mayor clearly abdicates his position.
* There is panic in the streets.
* Several nobles have gone missing and were captured by the succubus under Aberian's Folly.
* Vampires are revealed to have infiltrated House Mhartis.
* The PCs learn about the schism within the Council.
* Some of Chammady's more powerful minions (Avahzi, Aberten, Crosael, and Zol) die.
* Vassindio Drovenge dies.
Have I missed anything?
Umm ... the PCs making 9th level, and picking up some needed magical equipment.
Fair enough, but that's easily accomplished by pretty much anything I swap in. Nonetheless, a good reminder, and thank-you for responding.
I did try something like this, although due to various other circumstances (Namely, this was my first attempt at running a pre-written adventure and I made some mistakes, as well as none of the players feeling very invested in the plot) I knew my campaign was nearing its end, so I may not be the best person to give advice.
What I did was have the Mayor's house explode the night after the PCs escaped the Asmodean Knot. What I did was have the mayor show up and ask for help while they were examining the contents of the Chelish Crux. This basically gave them the choice of which book to do next (They were pretty seriously overpowered for the module.)
What you could do if you have other adventure ideas to make up those two levels, is have the house explode and the mayor flee as a background event. Make it (relatively) easy for the PCs to figure out what caused it, but don't bother with the whole pit fiend in the basement angle. You still have all the panic and chaos, without even needing the PCs to go back and investigate if they don't want to.
The chaos in Westcrown could be an ideal distraction for the Dottari to make breaking into Delvehaven easier, or it could make things harder as they make an effort to better defend the cities notable sites.
As for how the party could learn about the Council of Thieves and all of its politics I'm not sure. Perhaps it could tie in with whatever you have them doing instead, perhaps one has a shady contact that tries to get their help with something and informs them of the situation, or conversely Chammandy sends an assassin after them that turns out to be rather weak willed when captured and spills the beans.
It doesn't hit all the points you outlined, but it certainly will help keep the tone of the campaign and the information in the PCs hands closer to what the writers intended.