There is no functionality for pre-filling the login form inputs. There are no plans for such a feature. You might want to take a look at our demo site and consider something like our Login As module which is just some HTML combined with a auto action for auto logging someone in. Your alternative is to make a Joomla template override for the login module layout and add your hardcoded values to the input there.

Where do I make the override and what would be the code to put as the override?

Suggest reviewing Joomlas documentation below regarding layout overrides.

docs.joomla.org/J3.x:Layout_Overrides_in_Joomla

Would the guest profile revert back to normal default when the guest user logs out or would it stay for the next guest to change to their liking?

It will not revert. That's something you would need to code yourself.