Put the other way round... the heroes have to become positively door blocking.
No, I agree that this quest is not a very good quest (book). Dead ends become death traps, so one must take care that there shall be no ends, only loops.
Additionally, even if the heroes took some mercenaries into the first quest, they would be long-since dead by quest 7, so they have to do all the search actions themselves.
Last, there are on several occasions in the quest book falling block traps that block the only door to a room. In quest 2, it's only with luck and pass through rock if more than one hero comes out of the quest. I mean, do they really expect heroes to jump the first square after a door, routinely, everywhere?
In the EU version, the door can be blocked, but the falling block trap is triggered by stepping on one field north of the door. So there might be some more heroes more coming out of that quest.
If I (ever) get around to redesign this book, there would be some things changed for sure.