If Barnes dies, leadership transfers to the next highest-ranking man on the team. That has been the case for me so far. Perhaps if you have no sergeants on your team it doesn't work. In my opinion, you've got to keep Barnes alive no matter what. Send your men off to do the various dangerous jobs, while you stay back running the show. That's what your job will be in real combat. You are more alert and a better shot than any AI. However, I'm not sure if the men can conduct interrogations.