The pushrod is a single wire. The case is there to keep the wire from flexing. Test it - pull and push on the ends and see that they're 100% connected.
Center all trims on the transmitter before binding. Get rid of any Sub Trim if you have it. All should be at 0.
If your servo driver has a meter, use it to measure the pulse coming from the receiver. It should be 1.500ms. The receivers do a dance when things are starting up. Is that happening properly for you?
What is "servo spline center?" I've never heard that term before.
The servos should be set up such that the control arm is 90 degrees to the body of the servo when the servo is commanded to center (1.500ms). If the control surfaces are not at the correct center point, then you adjust the clevis so that they are. If there's not a spline position that exactly matches the 90 degrees, then you get it as close as you can and adjust the clevis to make the surface neutral.
With an AS3X system you should not use nor need trims. The system learns what center is from the transmitter when you start up, or when you hold things slightly off-center position for 2 seconds.
I really don't like that old receiver design. If it were me, I'd toss that out and put an AR636 in.
Andy