One thing that can make an airframe pull one way in knife edge is the tailplane height.
Changing this can make a big difference.
You can use anhedral or dihedral on the tailplane to cheat a bit as this has the effect of lowering or raising the effective height.
The old Prettner Curare had anhedral to compensate for a tailplane that was originally too high.
The full size Hawk/T45 has anhedral on the stab and it knife edges very well as a model.
Moving the cg forwards or back can help the knife edge trim but that will ruin some other part of the flight envelope. There is only one correct place for the cg and that is in the correct place.
Using too much rudder can also cause problems.
Jim