Just reporting in that yesterday my group consisting of five players and six 4th level characters killed the Carrion King himself, next to the Maggot Throne room, without any breaks after the last day of march to the House. I can't believe their luck choosing exactly the right directions and happening to find just the right secret passages. I'm quite sure they haven't spoiled themselves, and also I have a habit of rolling perceptions secret so I believe it was just pure luck.
They leveled up twice on their way (using PFRPG medium advancement track), but didn't stop for rest, so the leveling never happened before they were standing in front of the King (as fourth level characters that is!). In that battle two PCs got killed and two others went down, but the last ones managed to finish the battle, revived their unconscious fellows and without any next thoughts ran as fast as they could back to their way and retreated to the mountains before NPCs could react.
So, I gotta figure out how the story goes on after this unexpected event...