Adam, I got your message. I don't have any more pics than what you have, and if I did they won't be of any help because those bumpers you bought from me (not this one pictured) also had a reinforcement bar. However they were not in this condition. They allowed me to disassemble the bumper assembly completely because the corners were not welded. In your case the corner pieces are welded to the bar, which means even if you remove them they'll remain connected.
That bolt you tried to turn and only spins - has another nut on the opposite side. This is only accessible if the bar is removed, visible in this photo:
That 15 or 17mm bolt and the one adjacent to it, more inboard, holds the mounting bracket to the center metal piece of the bumper.
Four tabs per side (on the metal strip/cover hiding two bolts) need to be unbent in order to free up the other two nuts/bolts holding the corner pieces to the main center piece. Once this metal strip (one per side) is removed, the nuts/bolts holding the corners can be removed and the corner pieces detached from the main center piece. However since you have the welding issue to deal with, that would be the first place I would work on.