Oh, fine, eschwartz, force me to reveal my stupidity
It converted correctly the first time in that my Kindle did receive an AZW3, but the epub was missing
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> (even though it validated) so the quote marks and apostrophes, etc. came out wonky, plus I had forgotten to add some custom css, so when I saw it, I mistook it for mobi
Still wish I could figure out why the Calibre dual-mobi doesn't work, though.