Is there an admin demo?
Not exactly, there is a topic on http://try.discourse.org with admin screenshots.
It is fairly easy to spin an instance up and kick the tires, with a $10 credit so the first month is free:
I used [Bitnami Discourse Cloud] instead.
: Discourse Cloud Hosting, Discourse Installer, Docker Container and VM
One click and it automatically sets up the server. 1 hour demo.
Full admin access in under 5 minutes.