send-to-Kindle does now accept dual-MOBI, but the downside is an apparent bug triggered by KF8, where Amazon cannot extract the cover properly to send a thumbnail. Either KF8 or covers, pick either or.
The cover is still in the book, though.
@quiris has written a simple program to extract covers and add the thumbnails to the thunbnails folder.
https://www.mobileread.com/forums/showthread.php?t=212242