For instance the security model for a browser should be ultra tight and protect the user from the site, but as a developer I'd want to access and modify my files directly through the inspector panel.
Another example would be the use of cache, where I want the minimum possible retention while a user would want the opposite.
As you mention, settings in the developer tools could allow a myriad of options to switch from a "user" mode to a "developer" mode. But honestly I'd understand if it happens to be easier to build two different applications, even just for keeping the "user" side code simple enough to make it easy to maintain and secure.
Then eventually bundle the two apps together if you want to keep the "tools" right next to the "viewers".