How can I change the default delay time before automatic shutdown? I don’t want to change the default delay time before shutting down all the time, but on occasion I want it to be 10 seconds and still show the dialog. 

The only documentation I could find was for scheduled shutdown, which is not useful in my case unless someone can show automation to get the current time and schedule it for now plus 10 seconds. 

Is this doable with a tool or script to replicate the warning dialog and 10 second timer?
There are a couple of workarounds for this, one of which I discovered by accident some time ago - though it doesn't give you 10 seconds, it's almost instantaneous.

If you set a shortcut key for "Shut Down…" the Apple Menu will add another option, "Shut Down" [without the ellipsis]. This one doesn't present that dialog at all, it just shuts down.

The same applies to Restart…





You can achieve the same by holding  Opt ⌥  whilst the Apple menu is open. All the options that would normally present an "Are You Sure?" dialog will be actioned without it - you see the ellipsis disappear with  Opt ⌥  held.