I have read this thread from the beginning.

I did not see anything about the hinges that could be the real cause of the problem.

Some principles I follow ___

1 - Use hinges that have negligeable friction. (pinned hinges)
2 - Make sure the axis ofrotationis in a straight line.
3 - Make sure the control surface fall to their limit of travel freely just by their own weight.
4 - Install control linkages that do not bend under compression (pushing).
5 - Assure these linkages (whatever type you use) can take the full force of the servos while in compression without bending.
6 - A bit of imagination to get the straigtest routing posssible.
7 - After linkage installation and connected to the control surfaces, moving the control surface at the servo end of the linkages should feel nearly no friction at all (you knew that and that is the reason you posted). NOTE: I am writing for all readers and in particular beginners.
8 - Same if moving the trailing edge of a control surface with fingers; the opposition to motion should be mainly the weight of the control surface rotating around the hinges.
9 - The friction in the linkages should be nearly negligeable.
10 - The linkages should have no twist in their function.

The above principles will also assure to have the best possible resolution of the servo rotation.

Do I have to mention that no slop should be tolerated ?

Have fun with setting this all up and ending up with good contol surfaces operation.

You have received lots of good advice on the linkage part of the control installation.

Ragards from Zor.