Brain dump of what I was thinking whilst reading your article ...

To make it work by only showing the content area would require more consistency between themes. I think this is something WordPress.org would need to start pushing as there would need to be an agreement to provide a standardised class or ID around the main content areas for styling purposes. Otherwise it is impossible for the plugin to know which parts of the theme to delete and which to keep.

However ... this is all somewhat redundant, as this was the purpose of the editor specific CSS feature. Themes can (theoretically) make the visual editor look exactly like the front-end design.

So perhaps the solution is to make a plugin which displays both the code editor and the visual editor at the same time. This removes the need to put restrictions on how themes work, uses the current built in method of handling live editable previews and allows you to edit the content inline rather than it literally just being a preview.