Thanks, léonie. Reverting to the originals helps. However, I have found that there is more to this.
As described above in more detail, my System Library on my Mac, set to store originals of photos and videos, seemed OK. Then about 100 thumbs went missing after running Repair Library and re-syncing, and Repair Library was not able to restore the thumbs.
A week ago, I unchecked "iCloud Photo Library" for this library. A dialog appeared, warning "505 photos have not completed downloading. If you turn off iCloud Photo Library these partial downloads will be deleted." I thought these "incomplete downloads" might include the photos which did not have thumbs. Maybe if the incomplete downloads were deleted, when I turned iCloud Photo Library back on, good copies might be downloaded. A strange part was that this library had always been set to "Download Originals To This Mac", and the photos missing thumbs were added to the library last spring. That seemed plenty of time for "downloading" to complete, especially since this library had full sized versions in the first place, before they are uploaded to iCloud.
I proceeded to uncheck iCloud Photo Library. The dialog again warned, I clicked OK. Then I watched the library. The count of photos dropped, but not by 505. I continued watching. After a few minutes the count dropped a few more, but still not enough. The process of deleting the "incompletely downloaded" photos took over an hour, during which Photos provided no indication that the library was being modified. What did indicate a process was running was Activity Monitor, which showed Photos taking about 100% cpu time. (With multiple processor cores on my system, Activity Monitor can report up to 800% cpu time. So "100% cpu" means a process is taking about 1/8, not all, of the available compute power.) When Activity Monitor showed Photos cpu use dropped to near zero, I checked the photos count in the library, and waited a little longer to confirm it was stable. I looked and found the photos which had been missing thumbs, were still missing thumbs. The photos incompletely downloaded, which were now deleted, must have been others.
At this point, in Preferences I checked iCloud Photo Library, so the library again was my System Library. on my Mac. Photos automatically began to sync with iCloud. That was about a week ago. Yesterday the sync completed. Counts of photos and videos agreed between Photos on my Mac, and Photos in Safari / iCloud. I was also pleased to find that almost all the 100 or so photos which had lost thumbs, now had thumbs images. Very good result, the way it is supposed to work.
Unfortunately, three photos and two videos still did not have thumbs. One photo I edited with an extension, and saved the result. The thumb was created and appeared. Then léonie suggested simply reverting the edited photo. I did this for the remaining two photos without thumbs, and the thumbs were drawn.
All photos now have thumbs. Two videos are still missing thumbs, the problem was to figure out why.
First I noticed that the two videos were only 1 second in length. These were taken by mistake. I thought I had deleted all like this, but here they are. When I mouseover the video, the thumb appears. With Get Info I inspect more closely. "No camera information" and "No lens information", as expected for video. What I did not expect was "23.98 fps". These are all standard video shot on an Olympus OM-D E-M1 camera set for NTSC. NTSC video should be 29.97 fps. I looked at older video clips in my library. Almost all are 29.97 fps, a few very near this value. The two videos which do not have thumbs have fps of 23.98 and 23.98,, and length 0.01 minute. I suspect problem s with the frame rate and thumb creation may be because the video is too short. The solution is to delete the too-short videos.
Syncing with iCloud created issues, then syncing with iCloud cured the problems. Kind of strange, but my library is now in good condition, I think. The few problems not due to syncing issues I think were photo files poorly formed due to editing glitches, or videos which were too short to have proper structure.