Hi Aprot! I just discovered the 13th death so I'll write a few clues to help you out!
I don't think we can spoil so read bellow at your own risk! I'll space things out for you
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1- You're mistaken about the 13th victim's identity. Ruppert says it's been years since she heard Amelia play, and that she disappeared "one day". So we can assume she's not present today.
2- You're correct about the suspicious death
3- The reason that death is suspicious, is that we hear a thunder, but nobody dies. John is still alive.
4- This means someone has died instead of John.
5- Instead of numbers (like John being 1) you search this person through a letter. (such as 04-KI-Z if that person was in the kitchen at 04).
6- This person has to be alive after Annie and die before John.
7- Searchable locations are always in bold. Before John dies, a thunder is heard "far away". This didn't happen with other thunders.
8- When someone dies, everyone forgets them. Only later, the next person to die remembers them.
9- John never remembered Annie. Who did?
-This is the last clue! Below is the asnwer-
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10- Search 25-WI-K
The person that dies is Katherine (the woman on the other side of the phone). Even though she is Annie's and Victoria's mother, she only mentions Annie (because she already forgot about vic). Later on, after Annie dies, she calls again. At first she seems confused about who Annie is, then she remembers it's her daughter. She still doesn't remember about Vic. This means she's the next person to die.
The thunder is far away because it happens at Wintercote and not at the Galley manor.