In A Separate Peace, how does Brinker get Gene and Finny to attend the mock trial? Why is Brinker so determined to investigate the incident?

After Brinker had set up the mock trial and had everything in place in the First Academy Building, he and three other students came to Gene and Finny's room to get them. Brinker declared, "We're taking you out." Brinker's associates then "half-lifted" Gene and Finny "half-roughly" and took them down the stairs and out of the dorm. Gene and Finny went along with this at first, thinking it was some kind of senior prank at the end of the school year. When they got to the scene of the trial in the darkened Assembly Room, Gene still believed the whole event was some kind of nonsense:

At the front of the room there was a raised platform with a blustrade in front of it. About ten members of the senior class sat on the platform; all of them were wearing their black graduation robes. This is going to be some kind of schoolboy masquerade, I thought, some masquerade with masks and candles.

Then the tone changed as Brinker set the proceedings in motion. Gene could not see how any prank could be funny in this setting. Before Gene and Finny truly realized what was about to happen, Brinker led the group in prayer, and it was too late for Gene and Finny to back out. Gene remembered the moment:

If when Brinker had said "Let us pray" I had said "Go to hell" everything might have been saved.

The trial was Brinker's final attempt to find out what exactly had happened between Gene and Finny in the tree. He had raised the issue numerous times during the year, in various ways, never able to let it go. Brinker was the "hub of the class" at Devon. He could not stand the idea that he wasn't at the center of events, that he wasn't "in the know" in terms of every aspect of life at Devon. He also seemed jealous and left out of Gene and Finny's friendship; he could not stop meddling in it. Finally, Brinker was terrified of going to war and attempted to exert some kind of control in his life as it spun out of control. Organizing the trial, taking the lead, and controlling the event were very much in his character. By focusing on Gene and Finny's relationship, he was momentarily relieved of dealing with his own pressing realities.