I got some feedback from our lead developer regarding the versions of adb and fastboot bundled in the graphical installer:
The fasboot version used in the install script is the one which is compiled during the build of the system, so the most appropriate one. It is embedded in the install package and explicitly used by the install script. Installing another version is useless: it won’t be used by the installer.
So it seems the only relevant part of the above script is the “Google USB Driver for Windows” update. The script is just downloading and installing it from here:
https://dl.google.com/android/repository/latest_usb_driver_windows.zip
But I am a bit confused. Instead of direct downloading with the link above, if you navigate to https://developer.android.com/studio/run/win-usb and then click “Download the Google USB Driver ZIP file” it isn’t an exact match? The actual .dll files are the same, but the dl.google.com direct download link includes this disclaimer in a NOTICE.txt file not found if you use the developer.android.com download:
The .dll files here are distributed by Microsoft Corporation as part of the
Windows Driver Kit (available at
http://www.microsoft.com/whdc/resources/downloads.mspx) and included here as
permitted by the Microsoft Software License Terms.
Whatever 
I guess since the actual .dlls seem identical we don’t really care?