Photos unable to consolidate referenced photos
For the record, although I was reluctant to try it at first, what eventually solved my problem, was to


relocate all files into the Aperture library,
completely delete the Photos library,
reimport the Aperture library with all photos now managed.


Steps 1 and 2 can run in parallel to save time. After doing this, Photos is now able to scan all 17,000 photos for faces and upload all of them to my iCloud Photo library.