Who would be a good peer mediator at Devon for Gene and Finny? Why? What would this mediator do?
A good peer mediator for Gene and Finny would be Leper.
Leper has proven loyal (he wouldn't turn on Gene during the interrogation of chapter 11) and he demonstrates impartiality for the same reason.
Brinker would be an option because he certainly hopes to let Finny know the truth for Finny's own good, but Brinker might show partiality. He wanted to be great friends with Gene and see the two of them (Brinker and Gene) go off to war together, but he didn't get his way and became a rebel as soon as Finny came back on the scene.
A peer mediator would have to encourage self-disclosure from both parties, and help them express their feelings. Leper is used to listening. He would need to be armed with questions from the administration of the school. As one with a sharp memory for detail, when either party demonstrated bias Leper could interject and correct misinformation. A final priority of his job would be to get them to listen to each other and to hear the care and love that comes through in their words because in spite of the mis-truths that have gone on throughout the story, they truly do love each other.