Since you are only allowed to ask one question at a time I edited yours down to just this one.
Boo Radley is responsible for saving Jem and Scout from Bob Ewell at the end of the novel. He had every intention of hurting or even killing the children in a crazy act of revenge against the way he felt he had been treated by Atticus at the trial of Tom Robinson.
As the final chapters of the novel play out, Atticus learns that Bob Ewell was found stabbed to death and he assumes that Jem must have gotten a hold of Bob's knife and struck him with it, but Heck Tate makes it clear that it was Boo Radley who came upon the attack, and stabbed Bob in order to save the kids. Bob Ewell had a sharp knife, but Bob was killed by a kitchen knife. Boo's actions that night saved the children and saved Jem from having to take an action against Bob that he may never have been able to get over. If he had in fact hurt Bob in order to save himself and Scout, he would have had to live with the guilt and the knowledge for the rest of his life. As Scout realizes at the end, Boo has been their friend for a long time -- mending lost pants, leaving treasures in the tree, and saving their lives. This final act allows Jem to continue to have his life and his childhood.