Here is a new version to replace that I posted a few days ago and includes the tweak to the Check Mobi options so that mobi files with invalid headers also show up in the QC results.
This version also has a new check as per the screenshot of "Check broken image links". This will allow you to find epubs that contain <img> or <image> tags where the image being pointed to is not present in the ePub.
This can happen quite often, such as:
(1) You do calibre epub conversion, and calibre does not "properly" replace the existing cover. So instead you end up with an inner html cover page that contains a broken link, because calibre has removed the old image but not the associated html. When you open the epub in the viewer, you see a big blue question-mark showing where the image should have been.
(2) Your original source prior to the conversion contained incorrect links - someone renamed the images without fixing the tags
(3) The images were not present when you added the html to calibre
From a Modify ePub perspective, I am considering adding an option which will allow you to automate removing the html pages for situation (1) above.
While calibre does have a remove second cover option in the conversion dialog, this both (a) requires another conversion, (b) I don't trust it to always do what I want since it cannot discriminate whether an image is a front cover or not, and (c) IIRC does not remove the html page, just the img tag. So you end up with an extra blank page in your epub which isn't desirable either imo.
Instead such a Modify ePub option will look for html pages whose body tags contain only an image link which is broken and no other text. If so it will remove the entire html page. Or so the theory goes anyways...
As always, feedback welcomed...