Admin App

Mapping your website to your project route is not a secure practice!
XAMPP supports virtual hosting, so there should be no problem there.

You could always use a subdomain, eg www.yoursite.com and admin.yoursite.com, if you want to keep them separate.
They could still share a CodeIgniter folder.

There is nothing wrong with having an admin folder inside app/, with nested Controllers folders etc.
You could setup routes to suit the URIs you want to see.
Not sure if that is what Wordpress does or not.
This is closest to what has been called "HMVC" in CodeIgniter 3.

There is also nothing wrong with having an admin folder inside app/Controllers, with some form of role-based access so that functionality is only usable by someone in an "admin" role.