What is decided to be the 'key' point of evidence. In the case at the of act 2? How does juror 8 prove that there is a problem with this 'key' evidence?

The key piece of evidence is the knife. Juror 8 proves there's a problem because the boy (an experienced knife fighter), not only couldn't have posed the knife the way it was used but wouldn't have because he knew how to hold and use the knife.