Outboard Throttle Completely Stuck

I have a 20 hp 1978 Mercury 200 that I am attempting to get into running condition. For some reason the throttle won't turn one bit, and for the life of me I cannot figure out why. I took off the forward part of the tiller handle and it twists fine, so I would assume something is jammed in the linkage or something of the sort. Has anybody encountered anything like this before? I have poked around for about an hour and cannot figure out what is binding it.

Addendum: Am I just ignorant and the throttle is not supposed to turn when the motor is not running for some reason?

When you say you took off the forward part of the tiller, do you mean the small knob at the tip of the grip? If that's what you are talking about, you should just loosen that knob enough that the throttle will operate smoothly. If that isn't the problem, try removing the engine cover and disconnecting the throttle linkage at different points to help identify where the problem is.

The pictures I just added on this should help to clarify what I am talking about.

Clean any dried grease and dirt out of the gears marked in green in the pic. Follow the shaft (yellow arrow) up and disconnect the link from the upper end of the shaft and then see if the shaft and gears will rotate. Have you tried to open the carb manually to make sure it's not seized?

X2 on the above.