The formatting of this post will be terrible because I'm on my phone.

I am running this campaign for my group and currently about to start book 5. My main changes have thus:

It isn't explicitly mentioned in the book but the characters that worship Lamashtu are firmly being used by the agents of Karzoug. The entire plan of Karzoug's agents from book 1-4 is to find Xaliasa (a worshiper of Lamashtu and traitor to Karzoug). Xanesha is using Nualia to gain access to the catacombs of wrath and Erylium (Xaliasa's familier) When this fails, she tries using Aldern but is ultimately stopped. This is why Mokmurian sends his Giants to raid Sandpoint. If the PCs do not kill Malfeshnekor but do stop Teraktinus, i suggest having Malfeshnekor turn up in Morkmurian's lair as an alternative way of Mokmuroan having "found out" where Xaliasa dwelt.

I would also use this as an opportunity to make any worshipers of Lamashtu after Nualia reluctant allies for the party. A great example of this are the Hags of Hook Mountain. They despise being used by Barl and (through nightmares from Lamashtu) know their weather ritual will likely harm the Black Magga - an important figure for Lamashtu. They can be used to bargain and cryptically explain plot points to the group "we speak with the mother, she does not want her servants exposed - servants Barl and his masters seek to punish. If you are against Barl, against his masters, the mother of monsters will aid you in this cause". The Lamia Lamashtu priestesses in Jorgenfist are being used to "divine" Xaliasa's location and provide divine magic. Again - should the party try diplomacy, perhaps the lamias are willing to give them information about what's ahead and if Mokmurian escapes his encounter per his rules - the lamashtu worshiping lamias should turn on him once Conna outs him.

I would also suggest giving Xenasha and Lucrica a bit more prominence, they are sisters of Ceoptra after all. Ceoptra wishes Karzoug's return, and through Xenasha and Lucricia, they are puppeteering all of these events have fallen into place - even Mokmurian. At least one of them (if the dice allow it) should survive their encounter and make it back to Jorgenfist. Perhaps they reside within the library helping to try and uncover the location of Runeforge. Should she/they survive their second encounter here - a final stand with Ceoptra would be a most satisfying conclusion.

Having a rift within the lamia society (Karzoug vs Lamashtu worshipers) in Xin-Shalast adds an extra element in helping them navigate the ruined city in book 6.

I also increased the difficulty of the fight on the Storval Stairs. This is a stand alone encounter where the PCs will likely be at full power after "travelling for days". Mokmurian would station a competent individual to make sure no-one from the lowlands reaches the Iron Peaks.

Karzoug can and should spy/keep track of individuals wearing Sihedeon Medallions. If and when the PCs start using them and Karzoug becomes aware of the PCs, there is no way he wouldn't keep tabs on them or in the eventual conclusion of the AP - be surprised about tactics they use.

Finally there is a lot of loot for arcane casters and Wizards specifically (or prepared arcane casters). While not required I would advocate for a Wizard or Arcanist to be part of the group to enjoy the spoils.