It was Frank McCrae (Kate's father), who had - in life - been trapped inside of Maximilian; Frank's brain was used as the "A.I." for the big robot. Reinhardt did this because Frank led the
Cygnus crew in mutiny against him, and Reinhardt wanted revenge.
The angelic being we see is actually the soul of Frank being released and sent to his final reward in Heaven.
we see the Cygnus' robotified crew in "Hell," but why would they be there?
Part of Reinhardt's punishment, most likely. The humanoids aren't really there, probably just the bodies wandering around mindlessly (the souls having passed on to Heaven like Frank did). So Reinhardt gets to 'rule' over a 'kingdom' of empty shells.
Was Reinhardt and Maxmillian made into one being? Or....what?
Again: part of Reinhardt's eternal punishment. In life, Reinhardt trapped Frank McCrae inside Maximilian because the robot needed an A.I..