My proposal:
I'd make some tweaks to the setting--The Fall, The Titans, and much of the background would be the same, but I imagine fewer people surviving--perhaps 2 million, with crippled AI doing most of the 'grunt work' of building and maintaining habitats. The vast majority of people would be in the inner system (particularly Mars), or in the belt, making sure the robots don't get any funny ideas while they're out chomping up asteroids.
Things would be much more cyberpunk than Space Opera. I'd probably boil down alot of the luddite and conservative factions to a single "realist" movement--not a centrally controlled terrorist network, but a fanatical and devoted ideology that's also into blowing up "morgues" (backup databases), but is generally fine with augments that don't totally alter the human form. Uploading and backup is less casual--many people can't get over that *this* body fears death and dies, while a duplicate lives on in their place. Some realists go so far as to call these people Doppelgangers, and gladly space them if they can get away with it. Body swapping is still a popular alternative to space travel, and there is a bleeding edge of people who have abandoned 21st century notions of identity and self.
Many infugees were rendered insane when their backups were scrammed to orbital databases during the Fall. Too bad they can't always tell which ones...
Firewall might be more of a scientific organization, that occasionally needs muscle/expertise--their goals are essentially the same, but they're also looking for humanity's "next step" rather than just putting out fires. PC's would take the role of spies and fixers, with a potential gatecrashing subplot if that's where you wanted to take it.
There'd be only one Pandora Gate. It would feature strongly in the plot, although it's nothing but a rumor to most people, and it's location is a closely guarded secret. It's controlled by some not very nice people in the Jupiter neighborhood--and it's the only real reason to go out that far.
Luna is an industrial camp, where new habs, reactors, and ships are churned out in the L1 zone.
A massive power station orbits the Sun's corona, beaming antimatter around the system (ala, the Icarus array's Telematter system in Blindsight). Anyone who doesn't pay a subscription fee to the Consortium has to make do with He3 fusion and solar power.
I'd expect it to last 4-5 sessions, unless it really takes off.
Ironically, I'm running a post apocalyptic Pathfinder game right now...