In what ways do kindly slave-owners such as William Ford and Mistress McCoy help maintain and preserve the system of slavery?

Even though they might have been "kindly", they were still part of the system that perpetuated slavery. They did not try to help free their slaves: they just treated them better than most other slave owners. Their "responsible" handling of slavery could be held as an example to other slave owners, which only validated the system of slavery in many people's eyes.