I suspect the logistics of moving 5k refugees across hostile terrain would be a pretty formidable challenge. Perhaps the church of ST Cuthbert might push some divine help forward capable of creating food and water to assist.
Likewise, the governance of Sasserine or other settlements might push forces forward to provide physical security.
Possibly some good background for your situation would be the cities of Herculaneum and Pompeii. The citizens with the means and sufficient warning (ie nobles, religious caste members, powerful arcanists) would be able to flee in relative comfort while the average guy on the street would be found encapsulated in ash from the pyroclastic flows at the dockside (or convoy staging yard, etc).
After 3-4 days the party might return to a scene of anarchy as civil authority breaks down, food becomes scarce, and predators from the surrounding jungle begin to scavenge both live and dead citizens.
I think the players might find a hostile scene, particularly if the citizenry already know the PCs' failure put the populace in their predicament; conversely the PCs might be able to establish some sort of order and gain status if their failure is not widely known.
Assuming your Sasserine looks like the city depicted on Greywiki (15+k population, combination of oligarchy/democratic government), I think you can safely justify any solution you present to your players regarding Sasserine's response and outlook on refugees. If you want to present a humanitarian response, the various churches would probably be good catalysts.
Finally, please post how this plays out. I only run this one campaign and am always looking for ways to change it up from iteration to iteration. This is the first time I've read of a full blown eruption in the various posts. I'm interested in how you'll run this.
I would be curious to see how the world reacts to the PCs. If any details regarding their involvement leak out, I can't imagine things going favorably for them. Sasserine might take the refugees in but refuse the party admittance. Or perhaps the city is demanding the party submits to a trial. How do Celeste or Nidrama, good outsiders, feel about what happened?
I think that you're going to have a long trail of stragglers heading to Sasserine. The sick and wounded at the rear of this column. Bandits, jungle predators and so much more are going to pick off refugees. The refugees may even turn to banditry in order to try and acquire the things they need to survive!
Perhaps Hookface shows up demanding tribute, since Cauldron was in his domain, or else he'll attack Sasserine/the refugees/the party/whatever.