Justin Jackson- ⚡️ transistor.fmReedsy is the "Medium of ebook publishing." The PDF output in particular is excellent, but it also has the best ePub export I've found. This video describes how to convert it to Mobi.
Brandon Anzaldi- Software Engineer @ ToTheTensReedsy seems to be similar to what you need. They're designed for publishing, and have the ability to set you up with editing, design, publicity, or really anything you might need for working on your book, as well as a book editor which seems to be very similar to Medium's editor. Not sure about access control wrt main editor, but it's worth looking into.