Sorry - you can't unsolve it - you've given me the points now 🙂
Don't worry - I'll still try to help though. If I run out of steam (or you are not confident in my suggestions - I won't be offended) you might want to repost as the solved flag will stop some others from looking.
I think at this stage it is a good idea to have a backup of the library in it's current state - you have it on an external drive, but this is only a good backup if the drive is formatted APFS or MacOS extended (journaled). If it is, then leave it there, don't try to open it from the external drive, but keep it for a restore.
I've no idea why it is greyed out or has a stupid creation date. Never seen that before. Have you ever used dulicate deletion apps, or mac clean up/speed up apps?
A few things to try...
Restart your mac in safe mode, and try to open the library. Does the problem persist. If it doesn't there is a chance that when you restart normally it will continue to work.
Try creating a new empty library, importing some images and see if that library works correctly
Create a new user account, copy the library to that users pictures folder, and see if it works in that user account.
Finally try a library repair (Hold down the option key while starting photos and use the repair option). This is why the backup is important. Library repair can sometimes make things worse.
