Christopher Booker says that the hero in the "Voyage and Return" plot is often young and naïve. Sounds good to us. Then, for some reason, said young person finds himself in a strange world – like, we don’t know, helping a slave escape despite an upbringing in a system of rules and morality that condemns such action.
For a bit, Huck is super-excited to be in the outdoors again, which is to say NOT getting beaten up by his alcoholic father. He revels in the good times with Jim while they lazily float along on their raft. According to the "Voyage and Return" plot, the hero is never quite at home in his new world, which in Huck’s case seems to mean not the world of the "great outdoors" but rather the new ethical "world" of helping a slave escape. In this respect, it’s absolutely correct that Huck doesn’t feel at home, as he continues to question whether his decision was immoral.
Huck is increasingly bothered by that pesky conscience of his, as evidenced by his issues with the Royal Nonesuch scam and particularly the inheritance scam. He may not be too comfortable with this other world that exists outside of civilization, rules, and society.
This corresponds to the climax stage in the Classic Plot Analysis. Huck is overwhelmed by his crisis of conscience to the point that he is literally stopped dead in his tracks because of it. He has to decide: stay in the new "world" (which we’ve established is really a new mindset) or make a thrilling escape and return to the old one?
And here’s where we don’t quite fit with the "Voyage and Return" plot structure. Huck should escape from the "new world" of freeing Jim and go back to his comfy, cozy world of racism and slavery. But he doesn’t. Because he chooses to help free Jim, he’s actually leaving behind his old world and settling down in the new one for good. On the other hand, the fact that Jim was free the entire time might undermine the plot to begin with, since Huck wasn’t actually in this new world at all.