Is it just me, or did anyone else think about the gazebo? Because that's what it seems like. Regardless of the PCs tangent thinking, it is essentially a non-functioning part of the dungeon. TO have a realistic dungeon environment, there SHOULD be things that have nothing to do with the adventure. Unless you're used to linear convention adventures that is? And its perfectly ok for PCs to spend some time investigating that.
In I see all the fault on that of the DM. Two things happened. ONE,
This feels like a breakdown of the DM doing his/her job. There is no problem with the party splitting, that's there decisions. There is a problem with spending too much time on one set of players. There;s also a problem with the DM not moving the game along in a timely manner.
The major problem is that the PCs outsmarted the DMs and, instead of adjusting, the DM just closed up.
This happens to ever DM, and , going back to the original question, it takes experience to handle these situations. The DM obviously didn't think of this pipe as a means to get into the fortress. His goof, it happens to us all. How you adjust determines your skill. I think its great the DM provided a skill challenge of sorts to open the the valve, the key is, was it fair. If the initial description of the pipe is that it is used to flush things, it is reasonable to believe that there's a moderate chance of a pc reversing the opening. The problem occurs with what happens after the skill challenge. There was no finality to the encounter. The PCs need that in order to receive the que to move on. What I would have done. You attempt to jimmy the lock and realize tat the mechanism is beyond your skill to open. Back to the other players. I'll spend some time with the other players and go back to the original players asking them did they want to catch up. If they are still whining about the pipe Id explain how the roll was a roll to determine your skill to open it, and that there wouldnot be another attempt. They should catch up with the rest of the group so not to be shortmanned.
If the players have a good idea, then they should be able to explore the idea, no matte rhow it breaks the dungeon. HOwever, once you give this this opportunity then they should move on. I think the players assumed they'd gotten one over on you and even after failing at it, they played off of your need to appease them. Which, as a DM, sometimes you have to be cold and just say lets move on.
From the answers in the thread, I"m starting to notice how enworld is usually divided along party lines. If you're use to a more straightforward 4e style then the pipe should not be there, its useless to the adventure and thus useless to the story or game. Whereas if you enjoy the 3.5 style of gameplay then youre perfectly okay with elements that can be intereacted with in non-combat ways.