With the time left this day, the party could make it back to Three Rings by nightfall, assuming their trip through the swamp was uneventful. They could take the secret passage used by the gnolls, and bypass the salt slide. Monemvassia is about a 4 hour ride from Three Rings. If they were efficient, they could get their business done there and get back to Three Rings the next day. Then be back in the dungeon in the afternoon of day 3.
Frump says, "Mr. Causticus does sometimes come. He comes down the tunnel then knocks on the secret door. I would then send the gnolls out of the rooom and receive him. The goblins who normally deliver my supplies don't come into this room, and wouldn't know anything was amiss. They wouldn't come again for a week or so. So if you want to return to civilization, you should have time. Not to Orussus, of course, but Monemvassia. I don't know that you'd find anything in Orussus that you wouldn't find in Monemvassia. Want to get your friend raised from the dead, eh? I'm sure you could do that in Monemvassia."