Great post, Shanna!
You identify what is the crux of the credibility problem (well, one of many): that individuals are expected to convince others that the microaggressive interaction is part of a larger pattern of systemic racism. It's like trying to describe one fraction of a pixel in a massive image: no one can see it unless they were there, and it will always seem trivial to those not subject to it.
And there are far too many white ppl who will think they would've done the interaction differently--when you're always treated well no matter how awful you are, your experience tells you you're special. And no one is dissenting. Most white ppl cannot feel microaggressions, either--they don't have to. It's a privilege perk. right?
Researchers need to focus on collecting "relational big data" sets. (That's what I started doing in the field.) Those data sets would be irrefutable: you can see the racism iterated in the practices, over and over.
Once we see them at that level, it needs to be a national priority to do the kind of cultural work in white communities that challenges these practices head-on. They are lethal, over time. More than that, training that creates communication structures around interactions in professional settings that keep the interaction focused on the customer have built-in accountability. It's possible to address those practices with a supervisor, if the training has been done.
Anyway, my comments on your posts are always too long! Thanks for another accessible, important post!