The box.

[fixedG]

It's probably been suggested before, but I think Bree's dad's box of stuff is crucial to getting her back. She's not going to trust you guys as long as she sees you as captors. If she'll believe that her dad really wrote that note and she really made that collage, those are things that can't help but influence her back to her old self. The Order killed the guy. If she's so insistent that the Hymn of One/Order is looking out for her best interests, why would they kill her dad who she always cared for?

Show her the collage and letter. It's got to be more effective than getting advice from people she thinks kidnapped her and people she doesn't know on the internet.