To wrap up my own part of this thread (and perhaps to answer the original Op's question),
I talked to KDP on the phone. Engineering support got back and told me the PDF file had "transparencies". Recommendations were to:
A) NOT overwrite previous instances of the same file. Instead, create a fresh, new file.
B) Remove "transparencies" from the file.
I tried (A), even though it felt like a long shot. No change. Still failed, after upload, to process the file.
For (B), after much searching, I decided to open the PDF inside LibreOffice Draw and "Export Directly to PDF". File size was about 65% smaller than original. Uploaded it to KDP Print. A minute later, it was successful!
It appears they want VERY simple PDF files and won't do it themselves. So you must.