Is there a way to get minimal WordPress functions for iframed content?

If you don’t want get_header, then don’t call get_header(). It’s a simple as that, the get_header function is not where all of the WP core is initialized. The simplest way to do this is using the template hierarchy to create a template for a specific page slug or id.

From the WP Admin, create a page at the URL you want to use.

http://example.com/tos

Then create a template file that follows the rules for overriding the template for that specific page.

wp-content/themes/<your-theme>/page-tos.php

From there you can put whatever you want in page-tos.php and have full access to WP functions and API. And you can disable it by simply unpublishing the page from the WP Admin as well.