@
stumped - I suggest you post a scrambled version of one of the epubs you've split off, you do that via the
Scramble Ebook plugin. This will avoid any copyright issues and allow the bookmaker experts to offer their advice.
I don't know if the actual file names matter to calibre or the plugins, I suspect not. I remove covers, jackets and other artefacts with the Modify plugin, then I remove some more with the Sigil metadata editor, e.g. the <dc:description>, <dc:subject> entries etc. The only things I interested in having in an epub are the title, authors, publisher and date published.
So my knowledge of covers and such things is rather limited
BR