Quick suggestions: Have the PCs start at 2nd level, so they've got more survivability. Or if you want to make some more significant changes, give them Star Wars Saga-esque stats, with triple 1st level HP, and the ability to, as a swift action once per day, gain HP equal to their Con score.
A lot of people seem confused about how to react to the sign on the Pub door at the very beginning. I keep reading stories about parties who see the sign and think, "Oh, are we in the wrong place? I wonder how we get inside." I'd suggest just starting them inside the pub.
More detailed suggestions:
I'd suggest focusing on three main encounters, which form the meat of the session. I'd trim and condense some scenes, to end up with the following:
The first scene would be the ambush at the pub. This remains relatively unchanged, though you might have Torrent be a PC, or have her just give the PCs their mission without accompanying them, if you don't want an NPC around. I'd change Torrent's introductory speech to remove mention of the fire forest, and to focus on what will occur in the one-shot. Also, I'd make their 'escape route' much more clear: once they have the case, they go contact Herreman, and he can get them out right away.
After they survive the ambush, you have a short synopsis of their travel to the depository. Unless the first fight went really quickly, I'd suggest you not give them anything to do during their run. Just say people are panicking, there's a battle in the skies, buildings are on fire, and soldiers are running through the streets trying to get to the west wall.
When they get to the depository, completely excise the 'fake Rivereye,' and instead have them find the gnome unconscious and tied up. When awoken he tells them what he overheard, and thus directs them to Gabal's school (or the Elvish Ghetto, whichever of the two you'd rather run).
Whichever you pick is the second scene. Either works, because they combine roleplaying and planning with a fight at the end (though there's a chance for a peaceful resolution). Either way, I'd add Larion and the lantern archon to the Shahalesti in whichever scene you run.
After that scene, the heroes have the case and can leave. They go to Herreman, get horses, and ride out first thing in the morning. Give them a chance to find out what's inside the case; don't make it so impossible to decipher as it is in the adventure. Once they manage to crack open the case (just make it iron and locked, not adamantine), they find documents detailing how to create an eldritch machine, a magical superweapon that will let its controller use magic over vast distances.
The third scene is a combination of the gauntlet and the attack on Haddin's house. As the heroes are riding along, they run into the gauntlet, except here the bounty hunters are instead Ragesian soldiers (same stats, different uniform), and they are aided by the inquisitor.
How does that sound?
