Taking into account the adventure description, and considering that even though it's a book it will likely be divided into three chapters, my prediction for the adventure is as follows:
-Chapter 1 (1-4): We start small, traveling through the countryside to different villages, solving problems and gaining support while facing hellknights and some undead. The boss of this chapter could be a hellknight from Citadel Dinyar, or perhaps that comes later.
-Chapter 2 (5-8): We've just gathered support and descend on Elidir and other cities in the Conerica Straits. The boss of this chapter is surprisingly Hedvend VI; he should be around level 9 or 10, so he won't be the final boss of the book. At the end of the chapter, we manage to take control of Isger.
-Chapter 3 (9-11): You know who's a level high enough to be a final boss? Illcayna Alonnor, daughter of Urgathoa, attacks with her undead army from the south to return Isger to a nation dedicated to the goddess. This is the only reason I see for including information about Urgathoa in the book. Another villain we couldn't face is the High Priestess of Mammon, since she's level 20 and so on.
An outcome I would wholeheartedly wish for is that Asmodeus continues to be worshipped in Isger. I don't want Isger to become Ravounel 2.0, and given the whole orphanage situation (whose children are now adults), I think it would be good if they worshipped Asmodeus in his "benevolent" form. It would be quite ironic, if in Hell's Destiny the House Thrune falls and Cheliax is torn to pieces, for Isger to be the place that preserves Asmodeus. It would be interesting, and there would be beef with Andoran, which would be nice.