Install Powershell script with WiX
Group Policy is the better way to go about it,
I think the reason that your call to PowerShell.exe is not changing the execution policy is because the cmdlet is set to prompt the user before changing the execution policy. The -Force parameter will force the change without an additional prompt.
If you are using PowerShell 2.0, there is a -ExecutionPolicy parameter on PowerShell.exe. Try something like this in one single custom action to run the script.
ExeCommand="-NoLogo -NoProfile -ExecutionPolicy RemoteSigned -File <scrptpath>"