How does Rev. Parris nullify John Proctor's testament? How is Giles' deposition turned against him?
I had to pare down the original question. The significance of Corey's testimony being turned against him is significant for a couple of reasons. The first is that it officially shows that there is a justice system which is truly inverted in Salem. Giles Corey wishes to posit an alternate theory of activity in Salem. He has every right to bring it up in the courtroom, where the pursuit of justice should be pure and untainted. Yet, Hathorne and Danforth are presiding over proceedings where justice is not the first concern. Corey's testimony is turned on him because he is unwilling to name his informant about Putnam's land. In this, Danforth uses the idea that truly innocent people "have nothing to fear in this court." It is a variation on the "either you are with us or against us" mentality. Through this invocation, it becomes clear that Corey's pursuit of justice is something that is not shared by the court, more concerned with making themselves look good and continue to empower those in the position of power more than seeking out justice. Corey's deposition being turned against him is a reflection of this tendency and its significance is that it officially shows how far off the justice reservation the Salem court has moved.