I'll answer question #2 :)
"Why does Heck Tate insist that Bob Ewell's death was self-inflicted?"
Heck Tate keeps insisting that Bob Ewell fell on his knife because he does not want to drag Boo Radley into the public eye. Boo has been out of society for far too long, and although Heck knows that Boo killed Bob for a good reason, he might think that the public will not. Or yet, they'll make him a hero and he'll be overcrowded with gifts from the neighborhood. Heck knows that Boo does not want to be made public, and is a very shy person, so he knows that it is best to just tell everyone that Bob Ewell fell upon his own knife.
~Hope I helped!~