Make sure
Spoiler Karliah and Brynjolf are actually following you - run around in the cell you're in now and make sure they're following.
Then try a few other non-destructive fixes. (All of the following must be done on the console (tilde key))
First, check the quest stage: getstage TG08B
Spoiler At this point, you should be on stage 30, objective - Locate Mercer Frey
If this is not the case, take a look at this entry on UESPWiki and see if the solution pertains to you at all: http://www.uesp.net/wiki/Skyrim_talk:Blindsighted#Mercer_not_there
Next, just try moving yourself to the cell you need to get to
Spoiler Console command: coc irkngthand04
Spoiler If you move to that cell and K&B are not present (and the scene which is supposed to trigger upon entering that cell doesn't start), then this is not the solution. Reload a save.
No luck? Then do the following CAREFULLY: Go up to the troubled door, open the console, and click on the door (the refID should be 0010DE05, I believe). Use the console command disable, exit the console, then reenter the console (NOT CLICKING ON ANYTHING ELSE) and use the console command enable. See if that works.
The door in question is not a normal load door, it is an activator which teleports you to the correct location. While ALL doors could be considered as such, this one is set up differently.
Other ideas:If you are on the right quest stage and the appropriate people are present, you can try resetting the cell you're stuck in. See if
Spoiler K & B will follow you back to Irkngthand Grand Cavern
(the previous cell), then use the console command
pcb. This will force the cell to reload (pcb unloads all cells from the buffer, except the one you're standing in). Then go check that dratted door and see if you can go through it.
Still no joy? Well, the only other option I can think of (besides forcing quest stages or using prid commands, which I really hate to do) is completely resetting the cell you're stuck in. Go to the previous cell, use the pcb command, and then use
Spoiler resetinterior Irkngthand03
This command completely resets the cell as if you've never entered it (be sure to use pcb BEFORE resetting the cell!) Now, the quest involved does not reset, but all the monsters, treasure, and triggers will. Not sure what this will do to the quest (not delving that far into it atm).
Take a look at: http://www.uesp.net/wiki/Skyrim:Blindsighted especially the discussion tab and bugs list. Not sure if any of it will help you, but maybe it will give you some ideas.
Hope this helps.