My solution to this issue was to create a custom plugin with copies of the core functions needed to be overwritten. I gave these functions custom names so I could call them from the template files in the Child Theme directory. Now I could add my own code and modifications and leave files with the core theme functions intact.
This creates some redundant code, but now I can use a child theme and ‘overwrite’ core theme functions until the theme author makes the original core functions pluggable.