You are only allowed to ask one question so I have had to edit it down. In Chapter 15 it is certain that we meet a very different Sophie compared to the last time David saw her as a young girl. What is revealing to my mind is the conversation she has with David, which expresses what has happened to her has a "deviant" and how her life has been lived. This comment from Sophie is particularly poignant:
"To be any kind of deviant is to be hurt - always," she said.
This expresses some of the pain that Sophie has suffered, and we go on to find out that she has been sterilised, which she thinks is a fate worse than death, especially because now she is in love with "the spider-man" and wants to give him babies.
However, perhaps the biggest change comes at the end of the chapter, when Sophie kills a Fringes guard to release Rosalind and Petra with little compunction. Note the final paragraph of the chapter:
Rosalind, and Petra too, watched silently in horrid fascination as Sophie scooped a bowlful of water from the bucket to wash the blood off her arms and clean the knife.
Having been treated so unjustly by the powers of Waknuk, Sophie herself has become harder and capable of violence. Very little appears to be left of the childhood friend that David said goodbye to so long ago.