Your analysis was in some ways more interesting the film. I liked your very alternate take on the scene in which Waltz attempts to save the would-be runaway, and your contrast of his and Django's differing ends. I saw it merely as clumsy writing resulting from weak characterization, but you make a good argument for the arguable merits of it.
Thus, the film is an abysmal failure at addressing the other ensemble of questions Wilderson delineates, the prescriptive: “How does one become free of suffering? [Those] questions concerning the turning of the gratuitous violence that structures and positions the Black against not just the police but civil society writ large.” [p. 126] By giving the story a revenge motive, Tarantino reduced the suffering to a personal level, a subjective violence that one person might do to another — kill the oppressor, stop the oppression.
S'true, but then again QT's answer to Jules' theological deliberations in Pulp Fiction was for him to decide to "walk the earth, and get in adventures," like a TV character he enjoyed. Tarantino's characteristically been far better at digging unflinchingly into situational dynamics, than in providing moral insights. On the other hand (and tangentially) the majority of QT's characters are so thoroughly, almost fetishistically, driven by pragmatism that the introduction of compassion becomes inherently interesting. Mr. White's in RD, Jules' in PF (and Pumpkin's seemingly sincere submission to it), the bond bailsman's in JB, etc. There's always these little threads of compassion playing a subtle and unusual tug of war with both the logic of pragmatism and the conventions of genre simultaneously. Morality in QT's films seems to be centered around self-sacrifice, mainly. But revenge and pragmatism triumph as often as not. I still love Butch's sequence in PF in part because of how amazingly this recurring tug-of-war between compassion and pragmatism peaks. Then again, I think Roger Avary wrote that part.
Anyway, this said, I agree Jackson's character is cartoonish in DU. I thought he was great, but you couldn't help but feel: "Aw, that's all they're going to do with him?" Missed opportunity. I'm not sure I entirely agree with your overall critique of Stephen's function, though ... something there's not sitting quite right for me. But I'll have to mull it a bit, nothing I could articulate off the top of my head.