Finished the game and here's everything I found after 3rd dungeon (which is.. not a lot, unfortunately!)
Arrow room skip
https://youtu.be/p_7ZMkfOPng
This chest is close to the wall enough that you can clip into the wall and just upwarp there, nothing super difficult. Playing in this room a bit, I wondered if I could maybe shoot the target from way back, on top of the panel that opens up after, but here's what I found out: I believe that the game unloads/stop updating everything that is not rendered. So shooting the arrow from there to the target actually did nothing. So I went to the furthest place I could find where it was still loaded and hit it, then tried going back to the panel quickly, but I realized that it didn't do anything. When I walked back closer to the target, then it enabled the short cutscene and the panel went up. Zooming in with R1 also will make stuff render/update from further away also. May be good to keep in mind for certain things later
I also found a frame perfect exploit by accident that has no use for now but basically, there is a single frame before the cutscene starts where you can shoot an arrow and it will come out from the point of view of the camera during the cutscene, and I think the angle it shoots off at depends on your view before the cutscene. I actually did that on accident the first time and was amazed when I realized afterward that there is only a single frame window. Could perhaps be useful for something? Highly doubtful but still
Next up is a skip for the room I call the "bullshit room"
https://youtu.be/4MZ9-xU2VCg Almost like the dev put that acute corner there on purpose
Banana key skip
https://youtu.be/BI50rrM20Tg uses 90 degree corner + a tall enemy. Pretty standard stuff
A skip for the room just before the prison room
https://youtu.be/MN3jAl_Xe0U I actually had no idea how to complete this room legitimately. There was no place I could go? Anyway, I got crafty and shot 7 arrows at that crush monster (the category of monsters that crushs you) which is the max you can do before he dies, then pushed him the rest of the way to the wall, and used him to clip and upwarp.
To finish off, a quick upwarp using an acute corner to shave off a few seconds of dialogs and running
https://youtu.be/-YGQ53GzE2k What is cool about this is that the last prison cell has an easter egg. It's basically a refrigerator npc that, according to one of the developers in an interview, supposedly contained a celery and a carrot, but nobody was able to interact with it and therefore nobody was ever able to confirm this. This upwarp however changes everything!
One last skip I forgot to record. The dark night of aphasia, (4th dungeon world) you can skip the first room. There are acute corners all over the place but none of them let you reach the loading point because there is a ditch in the wall in-between. So what I did is push the movable box against the slanted corners at the far end of the room, and basically created my own acute corners. From there, I think you can figure out what comes next.
A small bit of information that's nice to know is that there is quite a delay before you take the damage when walking on the green goop in 4th dungeon. (the water-like substance of that dungeon)
Oh, and in the world right before the final boss fight, if you jump down into the lava, it'll put you next to the boss entrance which skips a bit of walking. (also, notice how that jump behaves, try it a couple times, you'll see why I'm saying that. I'm gonna have to study that one. It basically pulls you straight down halfway thru the jump which is REALLY weird)
Unfortunately, no boss skips/quick deaths were found after Clarence, which kind of sucks. It is sort of possible with Reggie (4th boss) because you can get on top of the walls and get the door down, like with Snuff, but reggie doesn't really care and moves all over the place. Many annoying rooms in the 4th dungeon could not be skipped either. It's still worthwhile to search in those rooms however, maybe I could have missed some interesting details.
After this, I'm gonna take a break with this game for a few days to think of some ideas and brainstorm a bit with what we have and what we know. All the obvious skips are there on the t able and now we'll have to think way out of the box I think.
