Yes, I tested quite a few and I can open the exported photos in Preview with no errors. I had to put the file I exported to on my external hard drive because I'm running iCloud on my desktop (maybe that's a mistake too?) and there's not room to download the 14K+ photos on my desktop.
My iMac is on macOS High Sierra Version 10.13.6
iMac (27-inch, Late 2013)
Processor 3.2 GHz Intel Core i5
Memory 8 GB 1600 MHz DDR3
Graphics NCIDIA GeForce GT 755M 1024MB
Photos is on version 3.0
iPhone 8
Verision 12.0
Model MQ7T2LL/A
My confusion comes when I had just exported all of these photos from my iCloud library, which I can view in Photos with no problem, but could not import them in. Maybe I'm doing something wrong but I followed the steps and the steps make perfect sense. The error doesn't make sense to me.
I'm curious also is there a way to see how many of these 14K+ photos have already been imported into my Photos? I assume that if I can get them to import now I will get the message that at least a part of them are duplicates. I'm pretty sure some of them have been imported and are in my Photo library locally, not only in iCloud. My concern is that I will lose any if I delete them off of iCloud and stop using it.
In order to create the new temporary photo library in Photos and use it as my iCloud library the software made me designate the new library as my System Photo Library. I assume that means that iCloud is now not on my old (original) Photo library.
Thank you again for your help with this!