In my opinion, you need to say that a word causes harm to prevent someone from using it. What you've given me is a perfectly valid reason for not using a hard R, something I agree with. The issue with this is that -a doesn't carry that same meaning with it. To say that a black person could use the word, and it has a positive connotation, but someone who isn't black using it automatically gives it a negative intention doesn't make sense to me. If we assume that the reason it's harmful is that it brings back memories of that oppression, why would hearing it from another black person not also bring up those memories, especially given the case that some black people can be racist against black people?
It seems a much more plausible answer to me that the intention behind the use of the word is the most important factor. I doubt anyone saying that a white person rapping along to a song that uses the n-word is racist, but that wouldn't make sense with your reasoning. If she was sending the message to a black person, with the intent of belittling their opinion with the word, I'd agree that she was being racist, but it seems much more likely that she is using it in a similar way to what you described in your 2nd paragraph.
Also a little bit of a separate topic, I don't know why the fact that she's Asian is an important distinction. My history isn't great, but there isn't a history of Asian oppression against black people. Wouldn't that make her use of the word neutral, since the race she's from doesn't have a history of oppression against black people?
tl;dr: The important factor isn't the history of the word, but the context and intention behind it's use. There are many situations in which I believe a majority of people would find the use of the word acceptable, so clearly, it's not intrinsically bad but requires a certain state of mind to be bad. People who aren't black using the world is cringy, but not inherently racist.