The Instapaper test that I was running had just one section and three articles in both the website and Calibre generated versions. BACK worked as expected in the website version but not in the Calibre generated one. I've been trying to find a way to extract and compare the two mobi's without interfering with the content. mobiunpack.py v0.23 is as close as I've got but this doesn't extract toc.ncx. Does anyone know of a tool to extract the ncx as-is? Apologies if that's an obvious question but I'm still pretty new to this.
Incidentally, much as I like the look of the new 'Sections and Articles' list, I've found that if you click on 'View Articles List' at the bottom, then you can page through and navigate off to individual articles quite happily from there and BACK does return you to the correct point. The layout isn't quite so pretty but it might make life easier depending on how you navigate round your periodicals.