The hymnal Reformation Glory is not fully indexed, but when I try to view any hymn instance data to edit it, I get a "Page Not Found" error.
I'm not sure where the "Page Not Found" error is coming from. You should be able to click on any of the instances and see the information. However, we require some training in order to edit the information. The training takes an estimated 2-4 hours. See https://hymnary.org/wiki - Training Materials for Editors.