The Twist .40 just doesn't knife edge well. Actually, I will re-phrase that... it isn't as friendly as some other airplanes in knife-edge. It can do it, just takes a littl emore effort. Playing with the CG will make a difference but not completely eliminate all of the coupling issues. I don't know if the problem is with the wing incidence (I'd think that would cause a pitch rather than roll problem), the shape or placement of the wing, the position or shape of the rudder or what. I know fuselage side area will also greatly affect how well a plane does in knife-edge.

I have a basic understanding of aerodynamics but I'm by no means an expert.

In my opinion, the Twist is a fun-flyer rather than a 3D plane. One thing you can do to improve knife-edges is to practice applying aileron opposite of the roll it trys to do. Once you master it, you can get a lot of planes to stay in a knife-edge.

Sorry I haven't answered your CG questions completely. I'm at work but I do have notes on that stuff at home. I'm sure someone else will chime in before I get there and look at them though.