But why a standalone page? - I'm sorry I am missing something that is probably obvious to you, but I dont see it.
You want something that is exactly like a moodle front page, that contains all the automatic content of the moodle front page such as the course list, but you don't want it to be the moodle front page? hmmmm....
Are you accessing this standalone front page from within Moodle or is it to be a front end of a web portal that will eventually take people into the moodle courses?
If the first - just customise the frontpage.php layout.
If the second I'd suggest that the courses list is not there because the session key is not set in your standalone page away from moodle and so its not pulling all the courses from the
database. The MAINCONTENT will probably need to be part of Moodle to get the session variables and populate that MAINCONTENT place holder. OR you need some way to pass the sesskey and other data to your standalone page outside Moodle.