Certainly plugin writing could use more tutorials -- unfortunately I am not the person to write them, as I am too familiar with the codebase, so I cannot put myself in a plugin developers shoes -- contributions are welcome.
As for what the external API of the Boss class is, it is documented here:
https://manual.calibre-ebook.com/pol...book.boss.Boss
which is linked to from the tutorial on writing editor plugins.
You can run any python script you like in the full calibre environment using calibre-debug file.py
which can be used for unit testing your plugins.
I'm not particularly inclined to make the plugin loading code more complex/slower just to support github's inability to create top-level zip files. As far as distributing plugins using github is concerned, I suggest you simply distribute them as source, a user installing a plugin from source simply has to run (to install plugin)
git clone
https://.../whatever && calibre-customize -b whatever
and to update plugin
cd whatever && git pull && calibre-customize -b .