Q: Hours wasted, please help.

'The selected photos cannot be accessed because an error occurred while downloading one or more photos for a print project.

Downloading? Is your Photos Library an iCloud Photo Library? If yes, did you uncheck the option "Photos > Preferences > iCloud:  Download originals to this Mac"?  If this option is disabled, your Mac needs to download some photos for your book from iCloud, because not all original photos will be stored locally.

The error message indicates, that Photos could not download the photos from iCloud.

Make sure, that your internet connection is working reliably, no firewall or antivirus software interfering, and you should be signed into iCloud.  Then open the album with the photos you used for your book and try to identify  the photos, that cannot be downloaded. double-click each of the photos to enlarge them. Do they download from iCloud, or do you get a warning?

I did have  my storage optimised for the cloud, but have unchecked that box, for what it's worth. The project still won't open.

I wish there was an alternate way to open the project even if 1, 2 or more photos are causing trouble.

It may take a while, until the optimized photos will download again from iCloud.

You can force that by going to edit mode.

I'd try to open the book, enlarge each page in turn, select a photo on the page, double-click it to bring up the options and and select "Edit Photo". One of them might give you an error, if it cannot be downloaded.

Then use a different photo for that page.