I can appreciate your attempt at fairness in your approach to this topic. However, I think it makes an assumption that leans in the direction of sympathizing with A. at the expense of C.: an equivalence between their experiences that assumes an equivalence is required in your response.
I suggest that the experience of being a POC is a shaming experience on a minute to minute basis at an institutionalized level. That is, POC's know that we are automatically considered ‘less than’ at some level, whether it be a conscious or unconscious conclusion. This is an embedded, constant experience for most POC's: that is, no matter how well I perform or what I do, some white person--often regardless of performance and because he/she/they are white--will get the reward, whatever that happens to be in that particular context. Studies bear this out in certain arena. This is a fundamentally shaming experience: why is it that my higher performance didn’t get the reward and a lesser performing white person did? What is wrong with me that my better performance wasn't good enough? This is an all-the-time shaming experience for POC's built into the process of privileging white people, who receive these items as if they earned them instead of acknowledging their extrinsic, unearned advantage.
I assume the experience of being a black man--which is an experience I never have, as a mixed race WOC who isn't black---is one of being publicly shamed at all times, at some level or another. My perception as a nonblack POC is that my experience is most likely less stressful than his given what I observe, so if I am feeling shame, so must he be. I assume this because I have felt it for over half a century in my own life, so I generalize while stating I cannot know for sure. Our society shames POC’s at some level or another, to some degree or another, at all times due to our being embedded in this context. He doesn't need a viral video to shame him because the context by which our society has been instructed to behave towards black males is, at some level, fundamentally shaming all the time (not to mention dangerous). He's being put in his place all the time, on this basis. Most POC"s, regardless of gender, are put into place all the time simply by living our lives in a white supremacist context.
I agree that two wrongs don't make a right: why shame A. just because C. is shamed? Yet, many POC's (and I am one) have watched the same behaviors for years and, despite kindness and candor, nothing has seemed to work. White people still assume the position of power, whether it be philosophically, psychologically or physically. Of course, the first leads to the second which leads to the third; hence, for example, better health outcomes for white people, collectively, than all others and the fact that their bodies matter more: if her white female body is at risk, she knows she can leverage that power to get what she wants. Whether or not it risks a black man’s body seems irrelevant since it is part of society’s shaming process used against black men (and their bodies).
The oppressor group is responsible for the onus of the work because the oppressor group has power. You point out that people like A. don't have power--like CEO's and government officials--and I agree; this higher-level power needs to be shifted so as to create balance and true opportunity for everybody else. But A. has power as a white female in a context that favors her, and she should take responsibility for this.
To me, the appropriate stance will be that any person in that class of dominance understand that the onus for active, proper behavior is on them, because they have the power as individuals, not just institutionally.
Otherwise, POC's continue to be shamed just for existing through anywhere from minor assaults like microaggressions to actual murder. And everything in between, on a minute to minute basis.
Why ask the victim to stop 'shaming' the offender through viral videos when the option of asking the dominant group to STOP the CONSTANT, EMBEDDED shaming to POC's exists?
The onus should be on the members of the dominant group.