View is using original template rather than new custom template
After a rescan and flushing of caches, the views-view-grid-panel-pane
  is bold in the tab, however, my changes are not reflected in the
  preview.


The view's preview does not use template files from your custom theme. It uses the admin theme. 

Thus, you can add the template files to your admin theme. The Drupal default admin theme is called "Seven".

Another option would be to just go directly to the view page url or where the view block gets displayed and just ignore your view preview.