Spike, I have been uploading VALIDATED epub2s to the KDP platform since January 2012 (it was Hitch, bless her, who told me that would work), and I've never had one fail. I do use a style sheet and I do edit the code. You can borrow my style sheet from
https://notjohnkdp.blogspot.com/2013...yle-sheet.html
Though it upsets some of the regulars here, I don't use heading and paragraph styles in Word, and I do use an old version of Sigil (0.9.18) since it works just fine for me. I also use an old version (2007) of Microsoft Word because I'm not willing to pay the asking price for a more recent edition. The only reason I use it at all is that my editors expect it, and because I have set of WordStar editing commands that making using it easier than any other Windows program I've encountered in recent years. I can't imagine going back to the mouse and the backspace to fix things; one might as well use WhiteOut!
Word2CleanHtml dot com online does an excellent job of cleaning up Word's messy html for import into Sigil. (There are plug-ins that do the same thing.) I think you should keep plugging away at Sigil. It's a great piece of software. I love it almost as much as I love my DOS WordStar from 1991.