You're doing way too much work for no good reason.
See
this thread and
this one for all the details you've ever wanted to know on the different choices you have to get working cover thumbnails without going crazy.
For people re-doing an existing on-device library, keep in mind that there may be some caching issues involved too. Better delete the 'broken' stuff, then let the Kindle notice (meaning, if you delete via USB: eject & unplug after the deletions, but *before* sending the 'fixed' files) before sending 'new' files.