Can I launch the Halo 2 Launcher by copying the Halo 2 disk to a DVD-R and adding the media folder and default.xbe file? If so, do I overwrite the existing default.xbe of Halo 2?

Yes.You overwrite any new data that you want in the new launcher of halo 2 so yes you would overwrite the default xbe.And all you have to do is add to the media folder the stuff that you want such as maps.I hope I helped!