Nice to know that you got it working. I’ve started to include EpicEditor that will allow for a preview of the markdown by itself.
One thing at the moment that bugs me is that it still uses the watch system to rebuild the page, which to my understanding rebuilds the entire site if a file is modified. In the context of an editor, I think that the page could be generated from the markdown file and then passed back as a html file, without it touching the publishing directory i.e. use hugo to preview the markdown file.