conceptually this is very fun! i think you took a solid core conceit and explored it well. feels like a 90s point n click game scenario that would kill me til i ragequit. the bride soaking in the bath not knowing the final discovery is such a sad image. she's so close!
i think i'm not reading the intro text right. it describes the final discovery, then points you to the secrets table, but i'm not sure why - that discovery (understandably) is not one of the listed secrets. what connection am i missing there?
i'll echo some comments below - the way you cut off the letter doesn't quite land. it's like a GM-only joke that doesn't work for players lol. i know my friends are opening that thing minute 1 and i'm gonna have to grit my teeth and bullshit about it getting wet in transit or something.
high: the soul overlap in the lab animals adds a lot to the eerie tone and seems fun, easy, and effective to play. my favorite detail is the ant-like mice. i would've loved to see you explore that more with something grotesque - a thief who drank an animal's soul and can barely function, a cat with the surgeon's soul cutting open its prey, stuff like that.
low: ditto the person below asking for more contrast - the chaotic folder aesthetic is cool and well-realized, but all the overlap without any depth feels too busy imo. the map is especially rough although i do respect you sticking with the blueprint look for the numbering.
in-between: idk why, but the marination hint on a big dumb post-it note in this otherwise crumbling house felt so out of place and incongruous to me that i googled when tape was invented to see if it even fit the vibe. turns out: 1845! that's crazy. i didn't recognize your game, 19th century surgeons.