Harper Lee - To Kill a Mockingbird - Questions

 Did ever explain the title?  Atticus Finch says that it's a sin to kill a mockingbird, but he gives his kids permission to shoot at Blue Jays.  Should it also be a sin to kill a Blue Jay?  What did they ever do to deserve being shot at and mockingbirds not deserve to be shot at?

If it's a sin to kill a mockingbird, then does the title signify "to sin?"

Is the Tom Robinson case, the heart of the story, based on a real event in Lee's life or did she make it up entirely?

I always thought that Boo killed Ewing, but apparently Ewell fell on his own knife and accidentally killed himself.  Is this right?

Why is Killing a Mockingbird a sin? From Google

In this story of innocence destroyed by evil, the 'mockingbird' comes to represent the idea of innocence. Thus, to kill a mockingbird is to destroy innocence." The longest quotation about the book's title appears in Chapter 10, when Scout explains: "'Remember it's a sin to kill a mockingbird.