Tony,
I thank you for your response. I was afraid that what you wrote, might well be the case, but that leaves me with another couple of questions insofar as the 2 options you listed, "file>export or file>export unmodified original," and a variation on my original mistake, i.e., Adding the iPhone images to my LrC catalog via importing them directly out of the Masters folder in the Photos.app.
Firstly, when it comes to the 2 export functions the Photos .app has to offer, "file>export or file>export unmodified original." In doing some experimenting, I have discovered the when using the first alternative "file>export" all the images have been converted to jpg, and the total number of exported files is about half the number I come up with when I export using the "file>export unmodified original" choice. Is that due to the fact that the Photos.app in the case of "file>export" treats all "LIVE" images as only the one image that the iPhone landed on when it offered a preview of the shot just taken, where as in the case of
"file>export unmodified original" the Photos.app exports all of the images taken in each individual "LIVE" mode shot, as individual images, and on top of there being other miscellaneous files included in that export option.
Secondly, I have experimented with a third option, that of going back to a CLONE of the Mojave Startup disk that I created on January 15th and saved to an external HDD. It of course includes the entire content of the Photos.app from the macOS Mojave version. If I recreate my original mistake, i.e., and just go into the Photos.app Masters file and pull all of the images out that way and save them to an external SSD, would that save time and headaches? I experimented and did that by pulling all images by the years they were created, so then I still have all of the other FILE FOLDERS that the Photo.app stashes them in as well as the ACTUAL HEIC and or (in the case of the earlier images) JPG files that LrC is interested in. If I transfer the files that way, and save them in a folder entitled MASTERS on an external SSD, I believe that LrC will link up to all of them without a problem. My only concern is, since the SSD will be attached to my new computer, will the new "PHOTOS.app 16" see that folder entitled "MASTERS" out there on the external SSD, that folder with all of the other PHOTOS.app sub-folders in it, and automatically "upgrade" it to the new PHOTOS.app? If so this "work around" will be, as they say, " all for naught".
Anyway, I have 54,000 files here and the task ahead seems daunting.
Any thoughts?