Agh, I have no idea how to efficiently quote while on phone: this post is for the two theories above. I’ll cut through both of them simultaneously.
When we’re talking about alibis, you can assume that once someone was in a group, should they have left it any point, that absence would’ve been noted. If it was not, assume they were in the group for the duration of the interval. In other words, Kumasawa and Genji did check the mansion after locking it up together, and Genji was in the dining room cleaning it with others for the duration of that interval. I guarantee you this.
After all, someone just casually just not being around their group due to some arbitrary reason not even hinted in the narrative defeats the purpose of this little alibi shebang, doesn’t it?
I also saw it mentioned in discord, so figured I should cut through this as well: The only guns on the island are indeed only kept in the gun room.