Does iOS keyboard "allow full access" setting allow use of data from default keyboard? Does the allow full access setting allow third party apps access to what's been written previously using the default keyboard? 

Or can they only access data that is written using the the third party keyboard app?
The 3rd party keyboard app are restricted to access data entered only using the app and not be iOS built-in keyboard.

The apps are sandboxed and have access only to what’s typed using the keyboard. It cannot read what’s entered using either the system keyboard or other third party keyboards.