After resisting it for a long time, I finally followed Hitch's advice to upload epubs to the KDP. I have mostly worked in html, and if I use Word for any reason I convert it to html at the word2cleanhtml.com website. I have developed a fairly comprehensive style sheet, which I add to the word2clean file (which does require some checking). I then open the html in Sigil and build a good epub with cover, metadata, and TOC. I then upload that epub to the various retailers including Amazon KDP.
So I really never touch a mobi file at all, never mind Calibre (which, as others have said, won't make it past the KDP Kopz in some instances).