how does the return of tom to the story in these last chapters result in a shift of tone and mood?

There is a sense of the old playfulness back between the two but Tom also brings a certain amount of negativity.

By negativity, I mean that Tom is not a free thinker like Huck. Tom has conformed to southern norms more than Huck has. Tom cares little for Jim; his interest in freeing Jim lies in the adventure of it more than anything else. We really begin to see how Huck differs from Tom. To put it bluntly, Huck is just nicer than Tom.