Switch the glasses. When the Overseer is about to die hit Curnow with a sleep dart. Then pick up his sleeping body. A marker will appear for a safe place to put him outside of the building. In a dumpster to be specific. This rescues him.
I sleep darted both of them and put their bodies on the ledge. I could then take Campbell to the interrogation room. Then I could take Curnow to the "safe drop off (a dumpster). The advantage of this is that its close to the entrance to the backyard, so you can easily make your escape.