@Tex2002ans Since I squared away the publishing/imprint stuff I went back a final time to try once again to make the UUID the ISBN, just 'cause that was bugging me. I carefully followed your instructions once more... and this time Kindle Previewer converted to mobi, no problem. So I did do something wrong the first several times. (Note to self: do code in mornings)
Thanks again for all your help, DiapDealier and Turtle too.