Hi there! I was surprised to get a notification from here after all this time. The good news is that I do have a solution, but the bad news is that it’s not with Scrivener, so is probably of no use to you!
I’ve been iPad-only for four and a half years now, and I’m a full-time writer, so it was very important to me to have a solution for publishing to both ebooks and paperbacks just from my iPad. I love Scrivener and always have, but the iOS versions lags considerably behind the amazing Mac app, and it looks like the iOS version hasn’t had significant updates in several years at this point.
Accordingly, a few years ago I switched to Ulysses instead, which pretty much has feature parity across Mac, iPad, and iPhone. It’s a very different type of writing app — it uses plain text (Markdown) for one thing, and has far fewer organisational and brainstorming tools than Scrivener — but I’ve adapted to its simplicity and focus. I use external apps for outlining and such instead, notably MindNode (both mind map and outliner now), and GoodNotes for free-form note-taking and brainstorming.
I created the Ulysses equivalent of one of Scrivener’s SCOMP files, called an export style in Ulysses, to create gorgeous ebooks of my novels. You can find my write-up about that here. About six months ago I also created an export style that lets me prepare paperback PDF masters from Ulysses on my iPad too; the details are here. Both formats support front/back matter, at least to a basic required level. I also wrote an article about publishing paperbacks via Ulysses on the iPad, which includes photos of the resulting book.
There are still some typesetting/layout niceties which Scrivener supports but which Ulysses can’t replicate yet (notably custom-text running heads, e.g. for the author’s name), but I’m willing to make a few compromises in order to have an entirely iPad-based publishing workflow. I’ve also created a Ulysses theme (like an editor colour scheme) that I find conducive to writing long-form documents, which you can find here.
I appreciate that this is hardly the answer you must have been looking for, so apologies for that. Scrivener is such an amazing app and I dearly love it, but as someone who doesn’t use a Mac anymore, the iPad version just can’t quite meet my needs at the moment. I do keep it on my iPad, eagerly awaiting any future updates! Scrivener and Ulysses both support importing and exporting Markdown, so I could certainly switch back in future if the situation changes.