Thanks for clarifying, Hitch.
Suffice to say that any add-on KDP validation process should probably be checking for meta entries (epub2), or manifest properties (epub3) to see if the mandatory cover image is indeed present. And it should be checking the guide section for a cover semantic to see if an xhtml cover page is present that Amazon advises
against.
That's how I'd handle the cover page/image section of any KDP validation plugin I was creating for Sigil, anyway. I'd probably create an output plugin that did the checking and removed the xhtml cover page if present. That way content creators could maintain one main source epub, and could generate a KDP-compliant version on demand, so-to-speak.