Until it's released, no one knows for sure. I am jsut going by the info on the main website,
Taken from the main website:
The villain fled somewhere into Avernum, an underworld nation of eccentrics, misfits, and barely civilized monsters, most of whom hate the Empire. And that is where you are going. You must find Dorikas, while dealing with spies, betrayal, and assassins at every turn.
The enemy is waiting for you. You will only survive his attention with the help of these strange cave dwellers. If, of course, they don’t kill you first
Now this woudl seem to indicate that the avernites haven't gotten to the surface world yet due to the attitude of hatred, which was a lot less severe in avernum 4. As for Rentar and Ihrno being gone, it could occur before Erika was sent down, or she oculd be missing for some random reason...