Thanks for the help Tinynanami, I would've never guessed!
Although I did think it might've something to do with Wintercot, I'd never linked it up with Katherine.
You explained really well why it wasn't Amelia West, although that whole clue did sent me of the wrong path of trying to find a death in the far past (also because of the convo in the kitchen about that death.)
If you're curious, my opinion about the suicide of Annie is that it actually made sense to me. Not that I'm the writer or anything, but my interpretation was that she was trying to find a loophole. If the curse dictates that there's a thunderclap before the death of the chosen person, her reasoning might've been that breaking that rule (dying before the thunderclap) could maybe break the curse. So that would be my explanation.
Also I think the story wroks better without an explanation. It's the same with the original Groundhog Day film. It works better unexplained, makes it much eerier I think. Although I do get your frustration. The story has a bit of an Agatha Christie vibe, so the absense of a reveal does annoy me a little bit to, although the death of Katherine fills that roll a bit.
Either way, that's my two cents. My compliments for figuring out the last death, top noth detectivework!
Ps: what were your theories on that hangman game? It's a 7 letter word but it says 12, so i thought there might've been a mention of an animal at timestamp 12?
Hope you enjoyed my ramblings <3