If your engineers are positive is not between the fwd and aft quadrants, I assume they disconnected both ends and with the cables tensioned they ran them by hand to make sure they were smooth?
If so you can rule that out as the source of the problem, question was any work done on the tail rotor retension pilot, new liner installed? If very tight could make the tail rotor controls a bit more difficult to move and sends this back to the mixing system.
Failing that I would say your problem has to be on the upper deck.
We had a similar issue where the pilot in the hover moved the pedals and the collective would go up. Basically the same troubleshooting as your guys are doing. The mechanics told me they ended up changing the collective mag brake. I thought at the time strange the pilot would not know his mag brake was not working properly when the trigger was pulled. Changed that and the problem has not re occurred. Also on the C++