However, the last two steps were not needed, as the Disk Utility restore process created a real populated Packages directory (not an alias) and put mach_kernel there as well.
I assumed those steps were from DP# instructions.
I think there must be some problem with how you created the USB boot drive. I hadn't tried for the Mountain Lion GM before, but I tried it tonight. I did have to replace the Packages alias with the Packages directory, and I also had to copy mach_kernel. I don't have time now to do a complete installation, but I did run the installer until it got to the screen that says "Select the disk where you want to install OS X". I'm pretty sure it would continue on to do a normal installation. What happened when you booted sounds like what happens when you boot from a recovery partition.