How does the tone in the first half of the book belie or contradict the second half of the book ?
All novels that present a main character generally make us feel for the character. In 1984, we trust Winston Smith's instincts from the beginning. I actually believe the tone shifts twice, at each book, not just somewhere in the middle.
During Book 1, we are in kahoots with Winston believing that the Party is re-creating the truth and that is a bad thing. We are morally frustrated with the government. During Book 2, we move as an audience from just believing to knowing that the government is bad. We learn to know it so well that we trust there are good guys out there to overthrow the bad government. By Book 3, Winston is tortured and recreated to believe that 2 + 2 = 5. In Book 1, the writing tone presented is distrust. By the end of Book 3, it's all access knowledge that the government is bad. Our mood as readers moved from disappointed to disgusted.