Doesn't seem likely that something is missing. More likely something is broken or bent. Have you disassembled the T.O. shaft to make sure all parts are in tact?
I can see your video, looks like this is an indicator of the neutral position of your handle shaft (part #11)
I'm working on my Throttle Only button this week too - those "just scrap it and buy a new one" posts all over the place aren't very encouraging. So far I've repaired a broken (#6 equivalent), put it all back together, had it work perfectly on the bench, then stop working once installed.
This...
