sorry again . I might not have been clear.

I wasn't having a prob with accessing the slave. Instead I was merely wondering if I could make the files listed above all return 403 errors instead of blank page/its own custom error.

No you can not.
The blank page for one of the configuration file is normal because it is required to call it sometimes before that Joomla start.
If you add the same "access denied", you will kill your website.
As the sensitive information is not displayed, this is not a problem.

In addition, almost all the direct access to joomla page does not return 403 but a message that like "the access is denied" or empty index.html.

ahhh okies!

so there is no prob at all. thanks for the clarifications!