Thanks a lot, ixtab, the file does exactly what I wanted.
Sorry about the confusion. I'll try to explain more clearly.
I'm assuming the book is marked as EBOK (CDE-type, exth 501 in the metadata) and contains an ASIN (exth 113 in the metaddata) created by Calibre that is not the ASIN of the kindle store.
Calibre does this by default and it does not change the ASIN in exth 113, the ASIN will still be the same if the epub book is edited and re-converted to mobi.
1) FW5.1.2 without the thumbnail fix (the coverview.jar file you posted today): the KT behaves the same way it does with FW5.3.2.1.
What the kindle does is:
a) look for a thumbnail file in systems/thumbnail with the filename thumbnail_ASIN_EBOK_portrait.jpg where ASIN is the ASIN in exth 113 of the book's metadata (a long number like 75b15e2e-a8dc-4e61-b69d-4e3bba884d1d if it was created in Calibre)
b) If there is no thumbnail file with the filename mentioned in a) the kindle will try to connect to Amazon's website to fetch the thumbnail. If the ASIN is not an Amazon ASIN you will end up with a file thumbnail_ASIN_EBOK_portrait.jpg.partial in system/thumbnails and no cover in the cover view. This is what I call the "Amazon thumbnail bug".
What Calibre does to fix Amazon's thumbnail bug is to always sideload the cover thumbnail with the apropriate filename to system/thumbnails along with the ebook. Everything works fine this way.
Note: another way to fix this is to remove the ASIN (exth 113 not set,this can be done in Calibre by checking the "activate sharing" box in the mobi-output screen). Then the kindle will create the thumbnail from the book. However books without ASIN don't work in the export to/import from Calibre part in the Collections Manager and might cause other problems.
2) FW5.1.2 with the coverview.jar of the jbpatch 4.0 distribution
a) azw3 (KF8) files behave the same as in 1)
b) mobi (KF7) files will ignore the sideloaded thumbnail file from Calibre (so it is best to completely disable sending thumbnails in Calibre by patching the device interface plugin which is what I did when using jbpatch 4.0). Instead the kindle will create a thumbnail from the book file and name it thumbnail_xxxxx_EBOK_portrait.jpg. I don't know how the number xxxxx (e.g. 53705) is created, it seems to start with some number and then get increased by 1 everytime a new book is side-loaded. (I think this is the same behaviour as for a book without ASIN in Amazon's default firmware without patch). This number does change if the book-file is re-loaded (doesn't matter whether you re-load with Calibre or just replace the file in the documents folder), i.e. if the book file has been touched, the kindle creates a new thumbnail file with a different filename. As I re-load mobi-files several times while doing edits, I ended up with many duplicate files.
Again, for me the problem is solved with the coverview.jar you posted today, Iwas just trying to remove the confusion (hopefully I have not created even more confusion now
)