I had to edit your original question as it infringed enotes regulations on what an acceptable question is. Enotes only allows you to ask one question at a time. Please remember this in future. I have therefore edited your question to focus on one of the many topics you asked about: the character of Lindsey Salmon. I have included links below to the enotes study guide on this novel that you can use to find out more about the other aspects you asked about.
Interestingly, it is possible to argue that this great novel is actually the story of Lindsey's struggles to accept her sister's death and move beyond the shadow that this creates in her family and in her own life. As the second of the three Salmon siblings, she is presented as being very close to Susie, and so she is obviously deeply impacted by her sister's death. However, what distinguishes her from other characters is the way in which she refuses to allow her identity to be shaped by the death of Susie. Clearly, the way in which she is shown to grow, mature and develop is undoubtedly confused through the different ways in which her family respond to the death of Susie, yet she is shown by the end of the novel to have matured into a confident and independent young woman who is able to play detective and risk her life to find proof of the guilt of Harvey. She is shown to be given a happy ending. She never lets Susie's death detract from her own ambitions in life and because of this is contrasted to her mother, who is shown not to follow her dreams. Note how the ending that Sebold gives Lindsey shows how she has processed her sister's death and moved beyond it, whilst never forgetting her:
Samuel walked out to Lindsey then, and tehre she was in his arms, my sweet butterball babe, born ten years after my fourteen years on Earth: Abigail Suzanne. Little Susie to me. Samuel placed Susie on a blanket near the flowers. And my sister, my Lindsey, left me in her memories, where I was meant to be.
Note the way that of all the characters, Lindsey is shown to have the healthiest response to her sister's death, and at the end shows that she has not forgotten her sister, but is able to life her life free from the shadow that her death cast on the family.