Good job on putting this together!
I noticed that you use the same validation test to check if the SP2010 Pre-Requisites and Bin files have been installed. That test is:
Test-path "$env:CommonProgramFiles\Microsoft Shared\Web Server Extensions\14\BIN\stsadm.exe"
However, if this test is run after the Pre-Requisites have been installed it returns False.
It does return True once the Bin Files have been installed.
Is there any way to check if the Pre-Requisites were successfully installed, that does a valid test?