I think I've gotten a solid grasp on the story, and it's definitely top notch. Shortly near the end, I was checking out different weapons and something clicked in my head, leading to me suddenly putting all the pieces together regarding the original sin, Trau, and the Egoistic Man. I'm genuinely shocked how much of their story is told in basically just 6 items, the Land of Sin weapons for each of the girls, the Well Key, and the Dessicated Arm.
I really really wish there was a wiki or something with all the item descriptions...
Though there are two things that I haven't quite figured out. The first was, what was the time/location deal with the land of original sin? If I remember the conversation right, the Flutterer said it was a place that does currently exist, but there's a lot of stuff implying that it's some sort of "see into the past" type situation. Hell, the Secret Keeper is literally described as a lighthouse, and the Nor Lighthouse is more known for how Shin used it to gather light to look into the past. Yet, the past can't be changed, so us killing the Egoistic Man there wouldn't have done anything for Trau to be relieved about, and we can clearly gain gear and items from it too. But yet again on the other hand, we see the 3rd Experiment subject and read about the other two clearly failing, and then come across the Wanter of Help, which seems to be that exact subject that should be dead.
Is it a case like Bloodborne, where the entire land is some sort of fantasy land formed from the hatred and trauma of the people involved with the souls of those people being sucked into it, is it actually there in the current moment but because of stuff it was sealed away unless you access it through the right doorway?
If it's formed from the minds of the people involved, it would explain the contraction regarding the Blockaded Cabin, as the lore seems to be that the man was locked inside the cabin until Trau let him escape, but the well key says he blockaded himself in there out of fear. Forced back into that situation again, he chose to hide away in the cabin away from everything and never left.
My second question is, who was that non-hostile pest in the lowest room of the Research Lab? Was it the Egoistic Man? Down the ladder in that room looking over the water, there's a human-pest that doesn't do anything, but once you beat the failed experiment and talk with Shin, the pest is now gone. It isn't blocking any path to anywhere, and it has the distinct human-pest sprite unlike the other eyeball tentacle pests that block pathways. It's so unique that I can't stop thinking of it as important, but can't figure out how it might be important.
IIRC, that's the room where you get the Desiccated Arm, so I'd want to say it's the Egoistic man, but there's nothing to confirm that since you don't see any higher detail artwork to confirm if he has the same lock of blue hair, plus he doesn't react to you even after getting the arm. I thought it might be that one pest that attacks you alongside the Sin-mana pests when you chase down Shin, but there's so little to really support that idea that I can find other than "I dunno, where else did we ever see human-pests recently?".