Carissa spends the next several subjective days building a wall of sorts so she can keep all the considerations in her head at the same time. Here's how we might try to take Avernus; here's how we might try to use my compact and other forces in Hell to side with us in taking down Asmodeus; here's how we might finagle the release of Rovagug so there's a chance of survivors; here's how things might turn out if we go home and leave this to Iomedae.
To her enormous irritation, there's a strong argument for the threaten-Pharasma plan. It's not one anyone thought of, possibly because it doesn't matter to anyone but her. In any plan for a war with Hell or in Hell, billions of the current inhabitants of Hell will die. Keltham thinks this is good, since they want to die.
Carissa...grudgingly acknowledges that they get to consider that an improvement over their current situation if that's how they feel about it, but it's still an almost-maximally-bad outcome for them. The man she interrogated about rumors in Egorian hoped for Abaddon, and he wasn't wrong to hope for Abaddon perhaps, but that doesn't mean she'd have been doing right by him by giving him Abaddon. She'd have been doing right by him by giving him other options he in fact preferred to Abaddon.
If they get Pharasma to give them Hell, they can make things better for the entities in it, instead of giving up on them. If they do anything else, probably several layers of Hell get destroyed.
Devils don't particularly want to live. She can see why, now; it has to do with Aspexia's favorite concept, corrigibility. Something that really really wants to be alive is a notably worse slave than something with any such strong wants.
Obviously, a great horror was done, when they took the shattered formless soul of someone like Carissa and shaved away the part where she wants to live. But she doesn't think all the horror was done there. Some of it has not yet been done, and will be done when they in fact kill that person using the fact they no longer want to live very much as justification.
If they do the Pharasma plan, she puts on her wall, they should arrange to be able to fix the devils and the paving stones. It is worth some risk - not, in Carissa's calculation, very much risk - but some risk of losing everything, for the chance of saving those souls too.
She doesn't talk to Keltham about Cayden Cailean and Nethys, but her guesses about what they want make it onto the wall too. They encouraged her cult on purpose. Why? How does ascension even work, does it matter if she has a cult? Should she be encouraging her cult? Would Pharasma care who all the Lawful Evil humans think should be the Lawful Evil god? Can Pharasma even see that? Why would Chaotic Good want a Chaotic Good god eaten by a Lawful Evil one? What are they getting in exchange for that, or is it more that - only in that manner can any compromise at all exist between Hells Keltham will permit to exist and Hells that Asmodeus will?
What is Asmodeus, at His core? What are the most essential features? He's not, actually, the god of torture; are there Hells without torture He'd abide?
What is the minimum guarantee from Pharasma they'd need to be confident they'll get to keep building Civilization? Can Abadar be trusted to look out for that? If He's handed a specification? How could Abadar not know something important to His domain He's been trying to understand for a long time, what kind of limitation is that anyway? Is it fundamental? If it's unknowable to Him presumably He wouldn't be trying to learn it, right?
How important is Golarion in the multiverse? How much do they lose if it's lost? Are there alternatives to destroying it? Are there alternative distractions that'd let an ascending god slip through? ...Zon-Kuthon's in a vault, right? Can Iomedae be persuaded to let Him out? Is that even worse than Rovagug?
What would Hell be like if it wasn't stupid? Forget what both alien entities hostile to human values - Keltham and Asmodeus - want here, what would Hell be like if it was a place for Lawful Evil as it abides in human hearts? What would the place Maillol should go be, if it wasn't Hell? If he'd turn down Nirvana, not wanting to be smoothed away into something Good?
It is actually only the second most complicated Wall she's dealt with recently but for the first one she had Asmodia and Korva. They're thinking that resurrecting Asmodia, even if Hell let it go through, would be a giveaway about who Carissa works for now. Carissa is less sure that going to the Gardens is impossible, especially since they still should talk to Erecura about anything related to Pharasma-negotiations. Most importantly, Erecura stole divinity from Pharasma and Pharasma doesn't seem notably weakened by it. Is She a different kind of thing than everyone else? If so, what about Rovagug made Him a danger to Her? Was it something about prophecy not working around Rovagug? Can they get them some of that?