Fixing this without mods

[LJMRT]

How can I finish Trynity Restored without mods?
I tryed to script code to disable the gate but the guy wont follow me.
Karliah just left and Bryngolf just keeps telling me to go or that Mercer is out there.

[Nasty Peep]

Is console commands an acceptable method? It may involve skipping an objective in the questline.
It may be worth a try.
http://www.uesp.net/wiki/Skyrim:Trinity_Restored
This has some of the known bugs that the quest has and offers ways to fix it.