What, ultimately, was the crucial factor in determining the outcome of the trial?

The most crucial factor in the trial's outcome was the fact that the jurors had to find the defendant guilty "beyond reasonable doubt". Juror Eight had doubts, and when they were presented and discussed, the other jurors were led to look beyond their prejudices and come up with a verdict based upon the evidence presented. In the end, they could not convict the young man on the evidence provided. Note, Juror Eight's discontant with the trial, and his determination to make everyone look at the evidence also played a large part in the outcome. Without him, the boy woud likely have been sent to prison.