RE: sub trim question
Ideally your pushrods meed the arms on the servo and control surfaces at a right angle in the neutral position. Ideally you do this by adjusting the pushrod length and choosing the right spline on the servo. Sometimes the splines don't give you enough resolution, and then you can use sub-trim.
I suspect that in this case your rods are not the right length, and the servo has to be off-centre to get the control surface to the neutral position. This means you will have different response rates, depending on which direction you move the control stick. It also means that you could easily run out of servo travel in one direction. You will see that when the servo is near the end of its travel, it does a lot of moving for not very much movement at the control surface.
In other words, if the linkage isn't right, do what it takes to fix the linkage. If you feel that this is not possible for whatever reason, you can at least remove the arm from the servo and turn it round a few splices to make sure that you have all the motion from the servo.