Without testing, this sounds right except the TB installation will only persist if .qubes-dispvm-customized is present. Because next time user boots whonix-ws to run apt-get update, the TB installation will be overwritten by whatever is in /var/cache/tb-binary in whonix-ws - when a new dispVM is launched. If no changes were ever made to whonix-ws after this procedure, then .qubes-dispvm-customized would not be needed because qvm-create-default-dvm would never be run by dom0. And TB installation would have no reason to be ever overwritten. ???