What does the depiction of the manservant in "A Rose for Emily" tell us about the historical and social context in America then? How do the townspeople approach him? To what extent would you consider the story a critical commentary about Southern society?

Emily's manservant Tobe could be said to represent the servile attitude that characterizes the townsfolk's behavior towards her. The people of the town put Emily on a pedestal, treating her as a living monument to a supposedly happier, more graceful period in Southern history.
But their understanding of that history is as romanticized as their evaluation of Emily. They don't know the sordid truth of what's been going on behind the elegant, faded facade of the Grierson residence anymore than they know about the evils of slavery on which Antebellum Southern society was based. What's more, they don't want to know.
Ironically, given that he's an African-American, Tobe is complicit in this massive deception. In acting as Emily's sole connection to the outside world, he helps to maintain the dishonest facade (that "all is well") with his mistress and, by extension, the traditional Southern values she's supposed to represent. It's largely because of Tobe acting as the keeper of the flame that the full horrors of what's happened inside the Grierson residence have been kept hidden for all these years.
And yet the white townsfolk must also bear their full share of responsibility for turning a blind eye. In both cases, what this illustrates is the extent to which a servile mentality can corrupt one's soul as well as society at large.