I wish Amazon would standardize all devices/formats! If the publisher font is included in KF8 code, it's forced in the Kindle for Android app with no other font options. If Kindle Previewer is correct, the reader can choose from the publisher font or several device fonts on Kindle Touch. Previewer doesn't have any font choices (other than sizing) for the Fire so it shows the publisher font if included.
When you get a chance, I would be interested to know if that is the way it works on the Fire device. If the reader cannot choose between publisher and device font(s), then I definitely won't specify a body text font.
Many thanks to all who have commented!