He kinda lost me at the idea of navigating a WIP using page thumbnails on the left. (a) That's so 1988 (Clive Sinclair, I'm thinking of you and your Pipedream here), and (b) the page is not the appropriate document element for navigation — scenes can be as short as a sentence or almost as long as a chapter, and scenes are how I structure prose fiction. Indeed, a manuscript page bears no real relationship to a published paper book page, or an ebook equivalent.
Finally, as functional requirements go this is, shall we say, less than ideal: but let's not go there.