Answer
Restore the environment back to its pre-upgrade-attempt state first: remove the newly created folder, rename the original renamed-aside folder back to its proper name, and restore a saved copy of the original web.config file back into the IIS site's application folder. Stop TAS in IIS, then rerun the installer from that restored starting point — it should now complete successfully. Going forward, always take a genuine ZIP archive backup of the IFSTAS folder before starting an upgrade, rather than a manual rename-based backup, and unzip the new installer package fresh into its own folder to run the update from there, since a manually renamed-and-recreated folder structure is what caused the installer to fail to validate the expected directory layout in the first place.