The fact that this only ended up working when using send-to-Kindle, versus using Calibre to convert to 'both' format, leaves me wondering is there some bug in Calibre? Or is this a known black box issue that you only get to work if you use Kindlegen, with no other solution on the horizon?

This is a fine solution for now, but part of me doesn't want to be dependent on *any* of Amazon's tools if I don't have to.