Do the king and duke ever feel remorse for their actions? What traits do they exhibit? How do they compare to round characters like Huck and Jim?

No, they have no remorse at all. They are con-artists oure and simple. They also convey another look at Huck and Pap's relationship if you look at them closely...... the King completely dominates the Duke and convinces hi to do his bidding over and over again, in a way that similar to Huck's situation with his father.

Huck and Jim are round characters because they learn and change throughout the story...... the King and the Duke always act exactly like we expect them to.