Is it possible to permit the public to see all the thumbnails in the album, and navigate around them, but not to click through to see the larger-sized images, and download them?

What do you mean with "larger size" images? We have three general size terms: Thumbs, sized image and full (size) image.
https://www.zenphoto.org/news/zenphoto-glossary/

There are options to disable access/download to the full size images. If you don't mean those, whatever a visitor can see is technically already downloaded by the browser.

I meant sized image and full (size) image. I'm investigating if it is possible for anyone to view the thumbnails, but to limit by password who can view (and save) the sized images and full (size) images, without having to give them Admin rights

Take a look here: https://www.zenphoto.org/news/user-rights/