I’m wondering if you misunderstood what I was saying about su. I’m not talking about su issued as root, but about su as a way of becoming root. If you log in as myself, and then type “su” and enter the root password, you will have an environment in which runuser will not be found, because PATH will by myself’s PATH, not root’s. On the other hand, if you do “su -”, you should get the shell to set up root’s standard environment, and a path that contains /user/sbin, assuming that your version of Debian hasn’t been set up to make it impossible to start a login shell for root.
From the man page, and its presence in /sbin, runuser is only intended to be run by root. If you start an install script after su’ing to root, and it tries to use runuser, you can expect it to fail,