What the Emma doin'? - Gyakuten Live! EPISODE 1 PART 3

The prosecution has brought forward a new witness. They are not keen to lying and they are (tied for) best girl. Cross examining her will not be easy.
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I figured it'd be the diary, but not for that reason. I never would have figured that out without Ayumu's hint.
I'm starting to wonder if this is all just a big misunderstanding and there is no culprit at all.