Nanjo gave the most important purple truth of "Even if it is not me doing the check, no one will mistake the corpses.", so it's so hard to confirm whether or not if anyone is dead. People talk about corpses all of the time, but they could potentially be mis-identifying the bodies.
However Nanjo does also say that he kills only one person (presumably his accomplice), and that the accomplice is responsible for all of the other killings. It's not said in purple, so taken by that it means that the accomplice killed 12 people, before being killed by Nanjo. Nanjo also doesn't kill Eva until after that declaration, so from that we deduce that the accomplice is not Eva or Rosa. It ALSO means that the accomplice has to have been killed between Battler's death and Nanjo's confession.
Battler helpfully says "Dad will not kill anyone." and "The person who killed the six in the study and two in the guest room is the same." which means the first eight deaths (Kinzo, Krauss, Genji, Kumasawa, Gohda, Kanon, Hideyoshi, George) are all excluded because the accomplice has to have killed all of them since Nanjo has only killed one. Rudolf is also exonerated because there are deaths that occur after that point, and Nanjo cannot be responsible. That leaves the suspect pool at: Kyrie, Natsuhi, Jessica.
So one of those three needs to have a faked death! Kyrie's death is the most suspicious since Nanjo inspects her body and is the one to declare her dead. Natushi is declared as "killed" by Battler and "dead" by Eva, so she's eliminated from the suspect pool. Jessica is trickier, but Battlers says that "Me, George and Jessica were in the cousin room of the guest house." and "it is impossible to go to the main house without being found out." so I don't see how Jessica could have done it meaning we just have to explain Kyrie
All of the deaths apart from Natsuhi/Jessica are fairly easy to explain. That pair however runs into some unfortunate truths about how she didn't leave until morning. My explanation for this is that Nanjo told the pair of them he would fake their deaths to throw the culprit off, and then directs them to Kyrie to finish the job.
I was satisfied with this right up until the point where I remembered about the chain being cut in George/Hideyoshi's room, and I don't have a thought for how to get around that.