Yes effectively it is a seconardy rollbar with an extra main hoop.
It does not attach to the remained of the cage in any way. Similar concept to where you'll see a ute with an internal and external (in the tray) rollcages.
Those are not 2 extra bars...infact I'm trying to remember what the firetruck they are...
Yes I still run the factory seats.
The harness do sort of slip of the shoulders - but you'll also notice I have a cargo barrier ther and this has cutouts which help hold the harnesses in position ... and once strapped in you don't go ANYWHERE