kiwidude, I ran across something this weekend you might want to incorporate into Modify ePub.
Nook STR (and color from what I've read), doesn't recognize:
<meta content="coverimg" name="cover"/>
It insists on having
<meta name="cover" content="coverimg"/>
...with the name attribute first.
Sigil and apparently caliber epub->epub conversion will 'correct' the problem, but since Modify ePub can update the cover, you might considering checking/updating the meta tag, too.
Might be something to check for in Quality Check, too.
I've also started a thread
suggesting that calibre could modify that when sending to the device, since it's updating the image anyway.