How to make custom end user text in a form available for editing for a e.g. editor role?
As long as the string or text is passed via t() function you can override it using String Overrides module. Here is a quote from module's project page:

Provides a quick and easy way to replace any text on the site.

UPDATE
You can also use Variable module. Quote from project page:

... provides a registry for meta-data about Drupal variables and some extended Variable API and administration interface.
This is an API module so it must be installed only when other modules require it.