Thanks for your reply, jhowell – that sounds like a hope I honestly didn't even really expect. Does a Calibre plugin for the "MOBI-both" format exist (it doesn't show such a conversion setting for this – just "MOBI") or is it really too unreliable when trying to upload the results? Would solve my problems if this works ... I haven't tried the export with Amazon Previewer yet, but for my whole 500+ ebooks, I guess the Calibre way (or any other bulk conversion method) would be much faster and more convenient ...
By the way, just tried your KFX plugin half an hour before posting here ...
Hoped that maybe this works, but no luck.
Thanks again for your help!