Finally have the chance to pop in and respond a bit! Wow, awesome discussion you've got going on. It is excellent to see the compare-and-contrast of knowledge and experience you bring to the game and the world.
Let me try to tackle at least a few questions!
1) Of the underdark/Darklands races, what made you choose the dark folk? Was it something asked of you by Paizo staff, or entirely your own?
When researching for the novel, the dark folk caught my eye as a race that, while present, hadn't been explored in too much depth. It left me with a lot of room to explore and play around with Izthuri and her people, trying to find ways to make creatures that could come across as vile or distasteful in some contexts and making them more relatable.
2) Who or what inspired the Roper? (I have named him Garrulous in my head because I love him so much).
Oh, that was a scene I couldn't wait to write for the longest time. Again, as you point out, ropers themselves haven't really been given "personalities" or shown as much more than obstacles/monsters to be overcome. I wanted a way to really mess with Ondorum's perspective on reality, even if he's being enlightened by what would be considered an evil monster. (Love the name, Garrulous, by the way!)
3) Does Izthuri have any class levels, or is she just the dark stalker (caligni) creature? Might they reach class levels in the future, or does that run counter to your own stance?
She was brought into the story as just the dark stalker, yes. I'm certainly not against creatures gaining class levels, even if it's via a home-brew set of rules.
4) Did the npcs in the derro torture room survive? Did you have any ideas of what happened to them (half-finished or otherwise)? ... if yes, would you be willing to share?
I left their fate a bit ambiguous on purpose. Obviously a few perished or were well on their way to doing so. One or two might've wandered out of the warren, maybe even made it back to their original homes, on the surface or otherwise--but doubtful that their half-mad mutterings of small underground lunatics would be taken very seriously.
