So the plumber came today.
We all made a collective decision - the plumber, contractor and I (JEM was not involved but I had his vote) that the whole pipe that runs under the floor to the sewer should be replaced. How does one get to this point? I dunno really, but a jackhammer was rented, a pile of concrete was removed and this is what now exists in the pit:
The trench (as I call it) runs from the stairs across the room to the far wall. Once everything is inspected, they can fill it in.