While I love Medium as a service, their editing capabilities pale in comparison to services with Markdown support. Their excuse seems to be that they’re trying to promote a unified style set, but utilizing Markdown does not preclude unified styles; it simply makes more typographic options available within a specified style.
I did devise a workaround, however, based on your truly brilliant idea of a Markdown editor for Medium — host the Markdown file with Gist and embed it using the share link of the Gist and Medium’s embed feature. See Let the trumpet blast for an example. Using this method will, of course, preclude the possibility of highlights for a story, but at least it allows for proper poetic indentation and spacing.