Alright, so I discovered an OoB in 5-1 a while back, but I forgot to post it here until now.

About 2 months ago, I would be messing around in 5-1 while Keith was examining the console, just hitting random buttons and whatnot. One time, after skipping the cutscene, I noticed that Keith was clipped into the console a little bit, but I didn't think anything of it at the time, I just carried on as normal.

About a week or so later (on Valentine's Day, funnily enough), I was playing 5-1 again, trying to figure out what I did to clip into the console like that. I eventually figured out that when Keith says "We might be able to open that area...", if you hold the button you use to ready your weapon (readying the Genesis also works), when Keith is done speaking, he will move forward a bit into the console. After skipping the cutscene, he will stay clipped inside, and you can move around.

After messing around with this for most of the day, I eventually worked it out to where you can skip opening both doors at the end, as well as going around the room at the end. I will include links to two videos I made here:

In this video, I'm playing on PS4 with an input viewer, showing how the trick is done.

This video shows the final version of the clip, played on PC, skipping both doors and the final room.

One final thought: I think this might be as far as one can go in 5-1. I messed around with trying to access the 9-1 fight area by going into the elevator at the end. While I was able to access it from OoB, I couldn't go up the elevator due to Quint saying something like "Hey, where are you going?" Going up the elevator into 9-1 would be a huge discovery.

Sorry if I was all over the place in this post, I just wanted to get my thoughts out there before I forgot about writing this.

Edit: Just wanted to mention, using the Genesis with default controls on PC doesn't work, as pressing the "C" key toggles it on/off. I personally use default key binds when I play on PC, so I can't say if using the Genesis will work if put on a different key/mouse button (unless maybe it's a frame-perfect key press). I do know that readying your weapon will always work, on all platforms, so that is always the safest option of the two.