how is huck finn shaped by society where it's legal to own another human being

I think that Huck evolves as a person more than changes. Huck always held a moral code that was separate from greater Southern society. At times his "Southern sensibilities" were at odds with what he knew was the right thing to do. In the end Huck does do the right thing in both your examples. Huck dislikes the Duke and King. He quickly acknowledges that their scheme was not mere trickery but was hurtful and dastardly. He not only disagrees with them, he sets out to stop them. Jim has become something of a soul mate to Huck. Despite their very different stations in life, Huck recognizes loyalty and goodness in Jim; he sees past the colour. Huck resolves to actually prevent harm to his friend despite societal expectations to the contrary.