As was correctly pointed out, guest mode is an ephemeral session that
should behave the same on all Chrome OS devices. Thus, the device owner
is not able to customize its settings.
If you want a configurable ephemeral session, "public sessions" is for
you. It is just like guest, but you can configure it via enterprise
policy. You do not need to push the stay awake extension either. You can
just configure power management settings to always stay awake.
"Public sessions" give you a browser window, just like guest mode. If
instead you want a single Chrome app to be running in full screen,
"single-app kiosk mode" is the right choice for you. Just like a public
session, it can be configured via enterprise policy.
- Bartosz