I can’t speak for how Mastodon handles these problems. (It’s awkward for me to set it up and give it a try with my configuration. I’ll get around to it one of these days.) There was that incident with Wil Wheaton, so there seem to be issues with top-down control, even if the top is distributed.
I’m certainly not saying that the bottom-up way I want to do it will be better or without problems, just different problems. What I want to avoid is a half-success where it works, but is immediately taken over by the alt-right and out-right Nazis. 
