How do you remove or detach that middle piece between the hinges that
slide in and out of the door? Is that connect by a rivit?

How did they construct these doors? build everthing inside, then welded together so now you can't get leverage to work inside the door chamber??

For example to remove the hinge from the door itself, how do you unscrew
the hinge attached to the door. Do you have to drill it through and replace it with new flush bolts?

 

If you're talking about the door check that keeps the door from opening too far, just remove the pin from the door jamb post and push it into the door. Reach into the opening and pull it out. There's a rubber grommet and washer and cotter pin on the end you don't see,

Barry