I'm not on Linux right now, so don't rely too much on the exact paths and filenames I give, but basically, it comes down to this.
I assume you run gdm because I haven't used KDM since ages
And I assume you won't be doing any administrative task under the same login as the simulator login
1/ Create a session script, let's say /usr/local/bin/XsessionSim:
I just hope that gnome-screensaver's ability to propose swithing back to the login screen is linked to GDM, and not to the gnome desktop.
2/ Create the window manager session. For me, it is in /etc/X11/wmsession.d/; just copy a previous one and adapt it so that it executes /usr/local/bin/XsessionSim. restart X.
3/ On the login screen (GDM), select your simulation user, and change the session to the one you just defined.
For the administrator however, the session will remain what you currently use.
You should be able to switch between users by hitting the screen-saver keys-shortcut, if there's one; if there isn't, just configure xbindkeys and add it as the first line of your session script: "/usr/bin/xbindkeys &" (probably).
If that is not enough, you'd best abandon the minimalist path, and turn instead to an adapted window manager, such as Matchbox.
[edit:]Now that I think about it, if you don't care for the screen-saver, you can just go ahead with xbindkeys, and configure it to launch /usr/bin/gdmflexiserver instead of the screensaver; you'd then remove the gnome-screensaver line from your script of course.[/edit]