Those tactics above work both ways though.
What is known about SM is that it attracts agent provocateurs style posters who are in for a bun fight or who genuinely "hate" everything/everyone. Those types can be posting as anyone and can create a history to support their chosen identity on a particular subject etc.
It's best to always be sceptical about a poster's identity and what anyone says about themselves on SM. No-one knows their motives.
You say there are black posters pretending to be white on MN, equally some posters may pose as a black person or pose as a NF racist or just post as if they're ignorant and dim, but might be stunningly bright. People's agenda is unknown as to what they're saying and why they're saying it.
None of it means that people don't experience racism. It's just the person typing the N word as you described, could be a racist twat or they could be looking for a reaction or they could potentially be looking to use it in the future as evidence to support claims of racism on MN. No-one knows for sure which of those it is.
Extreme of any argument always attracts people described above. The EU arguments - I was always called a Russian bot back in the day. Left for a bit - not because they'd won - just nowhere to go with people that don't understand someone can have a different point of view.
Similar has been attempted to happen on MN ie. shut down debate regarding H&M. However, when people are called racist, people will stick around to defend themselves against that accusation. It is only natural. So the discussions gets more heated when facts are misunderstood by so many that are offended on behalf of Harry.
However, as people mostly aren't disliking H&M for racism (I'm sure as you say - some may dislike them only for the fact that M has a black mum, not met anyone in real life that joins dots in that way, but you say you have).
None of us know for sure, so we stick to facts of what happened - and the facts, unfortunately for H&M do not come out on their side. They are both very entitled and self-absorbed people that expect to be forgiven any bad behaviour, but equally and hypocritically will not tolerate anything from anyone and cut people out left, right and centre. They were only here 18 months as a married couple and been gone just as long and every day it's longer. When they stop using titles and earning from the institution they despise, people will lighten up on the ridicule. Have the gumption to stand up for what they apparently believe would be a start. Bin the RF because they claim it's racist, but don't look ridiculous using titles.