With the tank empty. I aways cut holes in the elevons, and vertical fins to help with the CG.

There is nothing wrong with the mechanical mixer it works great. I have also mixed with the transmitter.

On mine I didnt use any type of spar, other than the short ones to attatch the fuse to the wing.
On low rate I set them up so that it wouldnt snap at wide open throttle when you used man up or down.

Hope this is some help