I am looking for help as to how to return the viewer to the same page where they came from using the Options/Gallery/ParentwebsiteURL.
Where in the core files can I edit or add the line

Options/Gallery/ParentwebsiteURL.

This has nothing to do with a "previous page". This is meant as the URL to any parent website if you use Zenphoto as part of a site that is e.g. powered by a different CMS.

To achieve what you like you will have to modify the theme used to do that this. You can learn the basics here: https://www.zenphoto.org/news/theming-tutorial/