How does the return of Tom to the story, Chapters 34-43, result in a change of tone and mood? -chapter 35

This is interesting. We would think that Tom's return would add a sense of levity to the story. On the contrary, Tom's value system is not the same as Huck's. Jim's fate hangs heavy in the background. The reader understands that Tom's motive for rescuing Jim is quite different from Huck's motivations. In this sense Tom represents the very society that Huck wishes to escape from. There is the for boding sense that Jim is not better off with Tom Sawyer involved.

Tom's return to the story leads to Huck's regression to old behaviors. These chapters are filled with fun and games, and the progression we've seen in Huck as a person fades away as he falls in with Tom's frivolity.