CodeIgniter: Multiple Applications, how to share resources?
This article helped me when developing multiple applications in the same install, I then used .htaccess for different applications and different domain names.
Codeigniter Wiki - Multiple Applications
You would probably have to experiment with the way the folders are set up and the way calls are made in order to access shared models controllers and views.
This post multiple sites, 1 codebase, using symlinks (with smarty) might help but might not be the way you want to do it.
The approach I would recommend for specifically the resources you listed, is utilizing an application "package" path at the same level as your application folders.
Go to the Application Packages section
just set your package path to "../common" or whatever you want at the same level as your applications.
Then That is a common place all your applications will look for these resources.