So for murder mystery as i like to call it, you have questioned everyone. Now you have to conrfont them.
Ami's diary, can be used on Dhena
Emmett's story, can be used on Erica
After this, Erica will confess to hearing a man's voice, talking to Ami in the hall very loudly. Since Emmett was outside covering the tree in chemicals, Erica's confession can be used to confront Ard, who said he didn't hear anything.
After all that, you should be able to confront Hana on the evidence that she had to have heard something, considering she was physically closest to the murder and others who were further heard the conversation.
If im right, that should lead you to a dead end and back to Morgana for the next step, which I won't reveal right now as the current step is kinda confusing as it is. Good luck, respond to me if anything doesn't make sense