Set screws in a nylon arm won't need any thread locker, they won't vibrate loose. As far as your rudder servo arm, if you can't get the splines to line up so it's centered you'll have to use the trim on your radio. If you have a computer radio it's no problem, just get the arm centered and save the setting so you can move the trim button back to center. If you don't have a computer radio you might have to live with the arm being a bit off as you don't want the trim button to be so far from center that you're almost out of adjustment travel.