How can I change the default delay time before automatic shutdown?

Solution 1:

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.