Looks really good. Like a simplified version of Scrivener, which is awesome. Would be awesome if I can import it to Scrivener and the other way around.
100% this. Word, or docx more specifically, is used for exchanging documents all over the place. It's readable (and writable) by Google Docs, Word, Pages, Open Office, and more I'm not aware of.
I do some proofreading, and I have done typesetting for a novel, and if the inputs I get are not in a Google Doc, it's in docx file.
OP has a great solution that focuses on markdown alone. And that's a welcome change!