OK, that was a lot (mostly in a good way). Spoilers follow, because I want to discuss some details of the game, so if you haven't played yet, don't read further.
The best moments were when things clicked together: reading Olive's journal and suddenly understanding the bones in the mess tent, or going back to the doctor's examination of Roscoe after finding Roscoe's letters.
I admit I had to fall back on a certain amount of guesswork: even now I'm not quite sure what the killer's motivation was (was it that Vivian found the letter with her paycheck and confronted him?), and I couldn't quite make sense of the timeline of deaths (I thought Vivian had killed the killer, based on having found the letters, when in fact it was the other way around). (Or, OK, it was both, but the final twist around that plot point wasn't something I saw coming, so I wasn't taking that into account.)
I also feel like I was missing something about the events that set things into motion: that is, who was the Mysterious Benefactor? What was their motivation? What the heck was going on with all that?
Anyway, that said, I really enjoyed this. Piecing together the information was fun, and--well before reading the author's note at the end--it was clear that these were all individuals with their own lives and quirks and flaws, and not some sort of archetype. They really came to life, which is so important with IF!
Now off to look at other things you've written...