Thank you for the file - I was a bit confused at first, but then I noticed under "Chapter Select" there was a new option for "SD-01 Unlocked" - which put me right at the start of the bridge.
Yes, I thought it would be an optional side area since i was able to figure out how to unlock the elevator before exploring more than 1/3 of the bridge. When I went down the elevator and saw the area, before i stepped out I realized "this looks like Story content" and tried to go back. When the elevator was marked as unusable, i thought i could reload my manual save made before going down the elevator. Didn't realize it auto-saved when i took the elevator.
I had managed to find a video of the ending, as I wasn't sure If I would get a reply - but very much appreciate your assistance.
It was much better to be able to finish what I had started for myself, and both my wife cried at the ending for a 2nd time. :)