Man, Miyu is an AWFUL detective. Who comes up to a person that doesn't even know there's been a crime, let alone that they're a suspect, and demands to know their whereabouts at that specific time and place and whether they can confirm any such alibi? Anyone that isn't an absolute moron (which obviously Koharu at least appears to be on the surface) would immediately get suspicious with that sort of questioning.
Wouldn't even be all that difficult to do better. I'd say, think up a story or two to tell about that night. Start talking about the more recent festival, then use the discussion of festivals to mention whatever cool or crazy thing you say happened the festival before that. Once the subject is placed to that festival it'd make a bit more sense to ask what she was doing at that particular time, and from there maybe lead into things like specific events or names of people involved. This girl has not planned anything out at all and is absurdly obvious about her intentions; if she's to have any hope of catching the killer, she's going to need to improve her game quickly.