2. Because of the Doctor's speech about how helpful and trustworthy Jim was, the people decide not to hang Jim and be a little nicer to him. This was very nice of the Doctor, and shows that not all members of society think in the same way. He showed the people that Jim had proven himself to be more than just a slave, and that they should treat him better.
3. The bullet signifies the consequences of romanticism, and its "death" essentially in the novel. It is a trophy of Twain and his realism against the world and its romanticism. That Tom kept it shows how some people never learn and still keep their old ways.
4. Huck is going to the territories, west of the Mississippi, because he doesn't want to be sivilized again. Huck has learned the evils of society and has decided to, in a sense, leave the world behind in search of his own world, away from society.
5. Huck Finn adheres to the realistic tradition of writing throughout almost the entire book. Even when it is highly unlikely that Huck finds himself at Aunt Sally's house and that Tom is there too, Twain uses this to strengthen the idea of realism even more and point out the flaws of romanticism. Huck is a good narrator because he is not too biased and is learning at the same time as the audience. His internal struggle with his deformed conscious gives the book a good conflict. He is also humorous and makes the story entertaining. Huck is truthful, and doesn't romaticize or over-exaggerate things. Sometimes Huck doesn't necessarily understand what is going on during some scenes because he is so naive, but it is still shown what happened and is perhaps represented through a different viewpoint, like when Huck saw the circus show. Having another character narrate the story would completely change the entire point of the novel, as no one else has a mind like Huck's and thinks like he does. The story wouldn't have as powerful an argument against society. An omniscient narrator would ruin the point as well, as everyone's (even society's) points would be presented, and it would make the story less personal.