You have to speak to everyone, examine Ami's body, and READ HER DIARY. I'm not yelling, but the diary is important. Then you should have to go back to everyone and confront them
Emmett's story -> conflict Erica
Ami's Diary -> conflict Dhena
Erica's confession -> conflict Ard
Ard's story -> conflict Hana
Erica will tell you she was reading in a tree. Emmett conflicts this as he was spraying the tree with a poisonous liquid that would've prevented Erica from being at said tree. She then admits to hearing a voice, a male voice in the hallway during the time of the murder. Since Emmett was spraying the tree, it can't be him, the only other male is Ard. This conflicts his story as he would've been very close to the murder scene. Ard will stick to his story and the MC(you) will be close to solving the mystery. You can then go back to Hana who was closest to the murder, but nearly everyone else heard Ami talking to someone loudly enough for her to hear. She will insist after hearing that the voices weren't recognized that it was an outsider. You then report to Morgana when you are ready. This should lead you to more of a one directional path to solving the mystery. If it doesn't, feel free to consult me again. I'm also trying to spoil as little as possible while helping you through it.
I hope this helps, good luck