I know this was 2 years ago so has the mystery been cut down from 7 to 4 chapters or is there some other plan?
Total chapters is now finalized at 6!
Chapter Five is going to be the last "real" chapter (and really just the second half of Chapter Four).
Chapter Six is entirely you making your accusation and then seeing which ending you got. There's going to be 15 endings, so the chapter will be kinda big, but if you only want to see one ending it probably won't take you long.
Because of how it's set up though, you can make a save
just before your accusation and play through that whole ending, then just reload that save and accuse other people to see how it plays out. I know not many people like having to play through a whole game over again just to see different stuff, so I've tried to make this as painless as possible in that regard.
Can you share any of the mechanics about the Investigation Board at the end of ch3, and how that may affect the story (if it does?)? Does trusting the right person maybe get an ally at the right moment, or being backstabbed if it's the wrong person?
The function of that board is largely to serve the same function as those recaps at the end of Chapters One and Two; the element of choosing who you suspect (or don't) will come up again at the end of Chapter Five when you make your accusation, but they don't have any bearing on how that character acts toward you (because it's not like you're running up to them and shouting your distrust).