The claim that Huck feels Miss Watson rightfully has on Jim is symbolic of Huck's view that society has a real right to dictate moral behavior. Huck, through much of the novel, feels beholden to the moral code of his society even while he violates that code. He sees Miss Watson's ownership of Jim as another example of the moral code which he accepts while making a decision, none-the-less to free Jim from bondage, breaking the law/moral code.
I agree that the river is the most important symbol. Try tracing the development of Huck's conscience along the river. It is fascinating to watch him make the physical journey at the same time as he is making the mental journey.
To develop #5, I would like to add that Aunt Sally likewise becomes a symbol of civilisation - with all its bad points that Huck wishes to avoid and run away from. It is this that results in the final words he gives us in the novel where he plans to leave and run away to escape being "sivilised." Note there are many different symbols representing the bad aspects of "civilisation" - the murder of Buggs, the Duke and the King and the gullibility of so many people they come across.
The Mississippi River is symbolic in a number of ways. The river represents a symbolic re-birth for both Huck and Jim. It can be argued that the river serves as a form of baptism. Both individuals contemplate their past experiences while on the river, sometimes through discussion, sometimes through silence. Another powerful form of symbolism is in the river's very name, 'The Mighty Mississippi". This is a powerful river that ended up having profound effects upon Huck and Jim. The both of them realized that they can never return to the places that they once lived as the people they were. The power of the river experience transformed Huck and Jim, both were 'raised up' from who they were before 'The Mighty Mississippi'became a part of who they had become. The river fundamentally changed the mindset, perspective, and soul of these individuals. It allowed them to explore the possibilities beyond the restraints that their prior experiences had denied them.
The Widow Douglass and Miss Watson are a symbol of civilization. They want Huck to dress, talk, and act in the manner that they prescribe, "She (the Widow Douglass) put me in them new clothes again, and I couldn't do nothing but sweat and sweat, and feel all cramped up... Pretty soon I wanted to smoke, and asked the widow to let me. But she wouldn't. She said it was a mean practice and wasn't clean, and I must try not to do it anymore...I couldn't stood it much longer" (Chapter 1). Besides just being strict about clothing and religion, they also own slaves. Jim is owned by Miss Watson, and Huck helps him to escape so he isn't sold. The two women symbolize many aspects of "civilized" society: they believe in religion, slavery, education, and clean living. Huck tries to escape all that.
In Chapter 7, Huck has had enough of Pap's abuse. In order to escape, he shoots a pig and fakes his own death. This is a symbolic act as well. Huck is literally "dying" to his old self, only to be reborn through his adventures with Jim. Although it takes awhile, Huck eventually comes to accept Jim as an equal and helps him try to escape slavery.
In Chapter 11, Huck and Tom come across a" wrecked steamboat" called the "Walter Scott". Walter Scott was a famous romantic writer best known for his novel,"Ivanhoe". The fact that the steamboat is "wrecked" indicates Twain's attitude toward romanticism. This is one of the instances which symbolizes the idea that romanticism is an idea that is past its time and is no longer useful.
The main symbol in the book is the Mississippi River. By the end of the journey, the river will have served as the vehicle for Huck’s development to maturity and for Jim’s transformation. The river will have also served to wash away Huck’s predisposition to use racial stereotypes and to heighten his perceptions of the ethical differences between deceit and honesty, style and substance, fantasy and reality. The river symbolizes freedom, in contrast to the restrictions and responsibilities Huck experiences on land.
"I was power glad to get away from the feuds, and so was Jim to get away from the swamp. We said there warn't no home like a raft, after all. Other places do seem so cramped up and smothery, but a raft don't. You feel mighty free and easy and comfortable on a raft."