It's possible to the Services layer of Drupal to get nodes, etc. but is there a way to restrict Drupal to being a Services layer, where the front end would be entirely something else not Drupal/PHP. Is there a way to prevent Drupal from lading anywhere and have only the non-drupal interface load which then communicates with Drupal through the Services layer?
Yes, this is perfectly possible. You need to install: CCK, Views and Services module. Then, in permissions set the "access content" permission to only admins (and editors and the likes). And expose your content via inaccessible views (views that no-one has permission to view). Those views will then feed the Services module.
Every page will render "access denied" except for the XMLRPC/services endpoints.
It is all rather clumsy, because Drupal has "presenting in HTML" hardwired into its heart. so you will need some weird configurations, but when done, it works very well.