<span>The informal language creates a conversational tone common with the oral tradition.</span>
Yes because if the author doesn't than the reader will never know if it was true or not
Answer:
He means that your own conscience must tell you what is right or wrong, and it does not matter how many others agree or disagree with you. As Atticus explains to Scout, this is why he is defending Tom Robinson, even though most of the town thinks he is wrong for doing it
the tone is assertive and defiant because by saying "I am the master of my fate; I am the captain of my soul" you are basically saying that this is your life and you are going against the order of things, especially if someone is controlling your life.