In
chapter 6 page 23 Maria says she "went further inside the warehouse but no one was there", but it's interesting that this wasn't a purple statement. It's entirely possible Maria was lying here and there was some additional person hiding in the warehouse, which could be the trick that resolves the purple statement that the warehouse remained a locked room overnight. i.e. mysterious person X was already hiding in the warehouse before they locked Natsuhi and Jessica in, then person X killed Natsuhi and Jessica without breaking the locked room. (Note that there was a moment in an earlier chapter where they were like "if it's not stated in red or purple, it's worthless" which could be hinting at Maria's lie.) As mentioned in my earlier post, Nanjo must have lied about sealing the warehouse afterwards, which means person X could have easily left and killed Rudolf, Kyrie, and Battler afterwards.
Of course, the problem with this is that there are no viable suspects for person X, since everyone alive is obviously accounted for and everyone dead was cleared by Erika's purple statements, and the red truth stated that Erika was not a culprit. If there's someone I would want to pick for person X, it would be Shannon (who would then naturally be the 7th woman in the study for the first murder). The biggest contradiction is that it's stated in the red truth that Shannon is off the island, unless there's a specific wording in the Japanese that could allow for a loophole. Obviously there are relevant details from the main story that expose some holes in this theory, but I won't get into those.