1 to the south that says they will use the password against od nua's followers. One in the North East that talks about the password. Choose that you are not a follower of od nua or whatever that text string was.
As far as I know there is no effect on doing everything besides stopping Thaos.
I saved pretty much everything but didn't like how the recruited paladin ended up of my companions.
Does anybody know where to find the bounty Songsmith Roska? I went to the area indicated and searched it. I also searched the entire dungeon.
I did not kill the guardians.
You need to talk to the ghosts. To do this you need to know their language. You can learn that as part of the main quest when you stop the machine that animates the dead.
You need to progress in the main storyline. As part of stopping the machine that re-animates corpses you learn the language you need here. I also got stuck at that point.