Does automator work when screensaver is active

That is very unusual behavior and should not be happening. Try restoring the firmware of the ATV by plugging it into a computer, opening iTunes, and selecting Restore.

Note you will have to re-enter all of your passwords into the ATV afterwards.

What is ⌃F9 meant to do? This worked for me when I wrapped it in a service:

tell application "System Events"
    key code 100 using control down -- ^F8
    delay 0.1
    key code 125 -- down
    key code 125
    keystroke return
end tell

You could try running the workflow with automator -v:

$ automator -v ~/Library/Services/lock.workflow/
Starting Automator…
Run AppleScript is running

Run AppleScript is finished

The Automator workflow has completed.

There are several issues with using Automator services to assign shortcuts to scripts though. See this question. If you run the service with a shortcut that has modifier keys, there might not be enough time to release them before the key code commands.

You could also use a script like this:

tell application "System Events" to tell process "SystemUIServer"
    tell (menu bar item 1 where description is "Keychain menu extra") of menu bar 1
        click
        click menu item 1 of menu 1
    end tell
end tell