@smokey @cn @eli ⚡️Eureka! Bullet 5 solves a giant missing piece.
That leaves one more mystery; when publishing a reply from a Micropub client, there won't be any way to select a given Webmention endpoint (micro.blog's endpoint or the destination blog's endpoint, if that destination blog isn't associated with micro.blog.)
Libraries like send-webmention try to autodiscover the endpoint so they'd never send a mention to micro.blog when replying to a post on a self-hosted blog.
I guess I could always send a blanket Webmention to micro.blog and it would work most of the time for how I'm using it.