What about Mr and Mrs Garner? how can we present them? how can we relate them to slavery as white masters to black slaves?

They were kind and understanding slave owners who allowed their slaves to hire themselves out, buy family members out of slavery, and to an extent choose their spouses. You can relate them to the many slave owners who didn't beat and abuse their slaves and contrast them with those who did. Slavery has never been "right," and it existed from the earliest of times, but every owner was not a bad master; there were many people who did the wrong thing and honestly believed because of the fair treatment they gave that it was right. Go figure.........