The setup wizard gives the capability to the end user to change the directory into which the setup will copy the files. How can I prevent this? If the user changes the directory, how can I know it, or how can this info be passed to the postsetup exe?
You cannot, from the wizard, prevent the user from changing the directory.
Second, as long as the postsetup exe is in the same directory as the application that is installed, the postsetup exe could read FULLPATH(CURDIR()) to determine what the directory is, and then update any configuration of the application files that is required.