No worries about the late response... especially considering this one is, too, heh
Good to know! While I consider myself no game dev, it's true I have some ideas coming up, so I'll be taking a few looks and it and see what would be possible (and reasonable) to include+expand on. (I won't delve into too much detail here, but I have thoughts of recreating a story I am writing as a VN, and the letter system could be useful to include "extra" updates in the original with worldbuilding that didn't advance the main plot... /thinking )
Speaking of... as I'm writing this, I just finished reading the extra material. Admittedly since I had very short play lessons to start with it took a while to get fully immersed in the mystery, but once I did... Hm, it's hard to write about mysteries without spoiling them for those reading comments before playing, but personally I'm leaning more toward the rational conclusion. Although I haven't had the time to elaborate even to myself why that is...
The choice of music nd where to place it was exceptional, too - as a hobby composer I should be taking notes. In fact, my only criticism re: audio and visuals would be the sprites for John, Paula and Rose. Hyanide did a great work with the sprites they provided, and the silhouettes fit the tone of the story as well as the sprites... But those three sets of sprites being a strange mix between them was slightly off-putting, and I do believe having them as silhouettes as well could have been a better choice. (That said there are more jarring examples out there - *cough Sengoku Gensoukyou cough* - and I was still able to take their scenes seriously.)
... I have no idea how to end this comment so: overall, 8/10 kinetic novel, worth trying to learn things from!