The problem with enforcing a saving throw every hour in game is that "time" can be quick or slow and it something the DM has to remember to do along with everything else.
It can also be silly if the PCs are sleeping and one fails a saving throw. Is he gonna wake up and attack his party or continue sleeping?
I am not fond of the DM hijacking the actions of PCs that way. It isn't fun for anyone. Making them suffer with damage or skill losses is one thing. Forcing them to act out of character is another thing.
With my luck the pacifist PC will be the one to fail his save. It is one thing to tell a player that he feels paranoid or feels and overwhelming desire to attack his fellow PCs but taking away total player control has never felt right to me as a DM unless the PC is actually possessed.
Furthermore these effects should probably only affect non-natives to the 9 Hells. Devils should be immune to these effects.
It's all well to have fire and brimstone affecting the PCs. How much should the environment have a detrimental impact and how much does it hamper game play. I don't like hourly saves. Time can move quickly in game too and then I'm stuck saying " roll a save....roll a save...." until either it's annoying or worse, someone fails and it is even more annoying to deal within the party. The end result is that the adventure gets bogged down in hogtying the PC who failed his save or having to deal with a combat when nobody wants it or is even prepared for it. Either way, it's a show stopper.
The paranoia should be roleplayed, but when he loses his mind, he will turn on the party potentially destroying the party, or at least damaging them.
That is not fun. Nobody likes save or die rolls and so I use them very very sparingly. Successfully saving one leaves a sense of accomplishment on the PCs. Just playing the odd is not a good idea. You are asking for 24 saves a day from each PC. That is just too many. It will bog down the adventure again and again and again. Eventually I'll just decide they've gone native and are no long affected by the environmental issues. This is a very bad idea. It's fine if you want to be a killer DM but those DMs generally don't last long because their games aren't fun.
I like the the mists are handled in Ravenloft. Yes they are poisonous BUT you can buy the antidote from the gypsies. And the mists come and go randomly at the DMs discretion.
That is how you handle paranoia in a gothic setting. In a demonic setting (especially one based in Call of Cthulhu) the idea is that the PCs are supposed to go mad. That is how all of CoC games end. If my players want CoC, we'll play that but generally D&D is supposed to be heroic fantasy. Not saying it can't have elements of it. But you are not going to have happy players if it becomes excessive. I think making PCs roll 24 saves a day is excessive. Not even CoC players had to do that.
I see you have updated your initial post since I wrote my critique.
Since this is what happens in the river Styx according to the Fiendish Codex II:
Any living mortal who comes into contact with the Styx must make a successful DC 25 Fortitude saving throw or be afflicted with temporary amnesia. The DC increases to 30 if more than half the character's body is immersed, and to 35 for full immersion. The amnesia lasts for 3d6 days.
You might want to use similar effects for the other rivers.
Perhaps this would be an easier way to say it this way:
The river Phlegethon has the consistency of lava; instead of molten rock, it is full of boiling blood, filth and bile. Any living being who comes into contact with the Phlegethon must make a successful DC 25 Fortitude save or take 2 points of hellfire damage per round of contact. The DC increases to 30 if more than half the character's body is immersed, and to 35 for full immersion. Characters who fail their save must also make a successful Reflex save (DC 25) or contact Filth Fever.
The cold, black, roiling water of river Styx releases toxic fumes. In a 30-foot radius from any point of the river (except for the gates, which are magically warded against its effects), living beings must make a Fortitude save (DC 25) or take 3d6 points of acid damage and must make another saving an hour later (also at DC 25). A successful save negates the damage and renders the target immune to the river's fumes as long as the target remains in Hell. Leaving Hell and coming back starts the process from the beginning.
Upon making a successful save, the target also gains a temporary +2 Circumstance bonus to avoid the other adverse effects of the river Styx.
Anyone touching the the river Styx must make a Fortitude save (DC 35) or become affected by a mind-affecting rage that renders you incapable of telling foe from friend and causes you to attack the target nearest to you for 1d4 rounds. Creatures that are immune to mind-affecting effects are not immune to this but gain a +5 bonus to the check. If there is no target available, you fight against an imaginary foe. Once a creature has been affected by the mind-affecting effect, it is immune to this effect for the next 24 hours.
Let me know what you think.