Warning Spoiler Plot Mistake in The Last Time I Lied

[Stan]

Near the end of the novel, Emma is remembering things that convince her Vivian is the murderer, including that Vivian said “let me in” instead of “let us in” when she tried to get back in the cabin after leaving with her two friends. But coming back to the cabin made no sense at all. She obviously had plotted to make it appear as if all three of them had disappeared (including leaving her sweater (which she was not wearing when they left))
across the lake. If she had come back after killing them, that would have meant she was the last person to have seen them, and she would have been the focus of the investigation. Moreover, she could not have known that Emma would not let her in and, thus, that she would have had to make an unplanned getaway, which would have been unlikely to succeed, given the remoteness of the location and the massive search that followed. The only thing that would have made sense was for her to have planned the getaway in advance, have all three girls disappearing together, and certainly not reappearing at the cabin, trying unsuccessfully to get in, and depending on Emma to lie and say she had not reappeared.