The only thing I hate about Planescape Torment is how the setting works. First of all the Planes themselves are shaped by what people believe. Personally I find this to be a major flaw in the setting, if the Planes just change to what ever people believe then that means everyone is right no matter what the truth is, and how would anyone develop their own belief if everything they had experienced in life is just how the people wanted to live and they could just form their own plane to live in? This all a major flaw in the philosophy what's the point of the faction war if they are both right? If the Planes are infinite factions can just leave and forum their own home.

What I think would have been a better interpretation of this is that the Planes them selves (the Outer Planes) are alive and only change to reflect what people believe, meaning the change is only an astatic and believe cannot change the laws and matter of the universe, after all if believe can change how the universe works then what's to stop a person of incredible willpower to unmake the Multiverse. The Planescape setting already supports my suggestion to a degree, for instance if people stop believing in a god the god dies but the Lady of Pain challenges this rule and just the whole believe changes the world system.

What I am saying is that the Belief physically changes the world thing brakes the setting if there is no limit to what can be changed.

But if it was everyone's belief than why didn't the nameless one just go around convincing people he was mortal and was also a saint so he wouldn't have to fight in the Blood War when he died. I still think that the Planes themselves have their own agenda and keep balance when things get a bit too out of check, see the Lady of Pain and the Transcended Order Faction. Meaning no matter how many people the nameless one could ever convince he was mortal, he was still immortal due to Revals magic and the Planes had already marked him for the Blood War.

There is a big momentum. You can't easily change things. It takes time and energy. Achali the Drowned could "change" a small part of her world with tremendous Gith Willpower but the plane momentum took over as soon she lost her focus.

There is a stable point in the planes : the Prime. This one can't be changed, it's the limit. It's the one that's the closest to our reality (well, to the Forgotten Realms reality). You can't change that plane with thoughts. The thoughts of people from the Prime are shaping the outer planes. And because they are loyal evil people on the Prime there are loyal evil baatezu in Baator. To destroy Baator you could have to destroy the idea of loyal evil on the Prime Plane. Not exactly an easy thing to do. You can slightly change the borders from the planes but not much more (the Curst swap is a Big Thing in the planes, create Adahn is a tiny thing as it won't affect the big scheme of things). The Prime is not much spoken of in Torment, a little by this guy in the Smoldering Corpse, but it's an important thing in the whole Planescape universe.

Some things are easier to change because they won't affect other things, other beliefs. Some things are almost in stone, because of the momentum of the planes, and because of the solid Prime Plane.

If you read Amber by Zelazny, Amber would be the Prime and the Chaos Courts would be the Inner Planes. Something like that.