Don't reverse the red and white wires on the servo extension.
Assuming standard (non-digital servos), the black is ground, the red is +5V, and the white is the servo position.
Connecting the red wire to the white wire will either fry your servo, or make the servo move to one end of it's travel and stay there. (I haven't experimented to find out which one of these two options actually happen )

To have a harness reverse the servo direction, I think it might have to have some sort of electronics built into the harness. I think your best option here (since you don't want to do mixing) is to reverse the mounting direction of the servo, or reverse the servo head. You should be able to set it up so that both servos work together to move the elevators without mixing.

Tom