There are several questions being asked in this post, so I'll stick to looking at some themes of this novel.
An obvious theme(s) is the focus on race, racism, slavery, freedom. Jim is a slave, and he is running away in order to obtain freedom. He is not in his fight for freedom alone, because Huck Finn decides to help him. Huck too is looking for a freedom of sorts. He sees a lot of hypocrisy in society. He doesn't see how a society can be just and fair while allowing a person to be property as a slave. This struggle between legality and morality is another theme present in the book, and readers see Huck time and time again eventually follow his own morality over what society says is correct and legal.
And for a starter I would go to work and steal Jim out of slavery again; and if I could think up anything worse, I would do that, too; because as long as I was in, and in for good, I might as well go the whole hog.
Alongside the morality theme is a theme about education. Huck is not a well-educated person, yet he doesn't need lots of societal formal education in order to know and do what is right. While I personally think education is important, it has to be recognized that Twain is making a powerful statement about individual education through experience.
Another theme is family and/or friendship. Huck and Jim are not brothers, but their experiences with each other essentially create a familial bond between them which runs deeper than a standard friendship. They may fight and argue from time to time, but they are there for each other as well.