We wanted to avoid needing to spell out diffing mechanisms for every conceivable format in the core spec.

The trick around this to technically remain spec compliant, while also not surprising consumers of the WebSub API, is to say that your WebSub-enabled URL returns only the latest item. That way you can send just the one <article> or whatever, and it still counts as "sending the full contents of the URL". In practice, it's fine to do this as long as the page you're sending has a concept of multiple items in a list, and if your consumers are ready to handle arbitrary numbers of items.

It worked for me, but of course I had to set the authorization endpoint in the settings back to https://indieauth.com/auth.

If you have the authorization endpoint set to the built-in one, then it would be kind of stuck in a weird state. Which brings me to another point. How do I know what the URL for the built-in authorization endpoint is? Right now if I change it to indieauth.com, then I have no way of knowing what to change it to to make it go back. Blanking out the value just stores a blank value which breaks it. Can you make it revert to the default built-in URL if a blank value is saved?

return get_option( 'micropub_default_post_status', MICROPUB_DRAFT_MODE ? 'draft' : 'publish' );

This doesn't work because the problem is not that micropub_default_post_status doesn't exist, it's that it does exist in the database and the value is blank.

Was able to reproduce this after carefully reading the other thread. The key is this: https://github.com/snarfed/wordpress-micropub/issues/104#issuecomment-359267767

If you save the writing settings page, it writes a blank value for micropub_default_post_status to the database, so then the get_option call later will return the blank value instead of the default.