[DiapDealer]

I can repeat your results with the bog-standard default Sigil epub2 (which passes epubcheck with flying colors before converting). After conversion with the Epub3-itizer plugin and reopening the new epub3, epubcheck complains about the missing title and the required missing (required) li element. I can see the empty ol tags in the nav.xhtml.

Those errors seem perfectly normal given the input. I'm just trying to figure out how there could be a missing closing spine tag that Sigil itself seems to ignore (and actually shows as being there), and Epubcheck would ignore as well.

I'm not ready to jump into the epubveri game just yet. So I can't offer any input there.

However: I CAN confirm that the closing spine tag in the opf is indeed missing when I open the converted epub outside of Sigil. The question is why Sigil opens such an epub with no complaints and pretends the closing tag is there?! It seems to be silently fixing it, but not flushing it to disk.

And another question is why the epubveri plugin can see the missing spine closing tag while Sigil and epubcheck cannot.

Just saving the epub in Sigil after opening it is enough for it to silently add the closing spine tag to the opf.