[azimuth]

Ok, so the "content_0_item_0" is the title page. Then I'd suggest adding the following in "<spine>" in content.OPF file as it typically is the first item in that 'spine' string. We are suggesting this so as to hopefully resolve why the thumbnail cover image is not rendering.

<spine>
<itemref idref="cover_image"/>
</spine>

In <metadata> almost every epub OPF file that I've glanced thru, it has the part you removed:
'<meta name="cover" content="image_cover" />'

**your OPF file looks much cleaner now (all of it is well compacted, and much easier to analyze).