Ringer ... I don't fully comprehend your explanation, but let's back up to the top of the swing and track the club CoG to Impact.
At the start of the downswing, the clubhead CofG on the "longitudinal gravitational axis of the club" is behind the shaft axis, and both are 'on plane'. The shaft axis and CofG axis start diverging when the lead forearm begins pronating the hand to square up the clubface for Impact. Pronation begins after the shaft is parallel to the ground.
This can be called "axial release" (as opposed to radial release), with the club rotating around the CofG axis .. that is the toe is closing and the heel and shaft are opening .. or rotating CCW for righties. Since the clubhead speed is increasing, the shaft tip will start to 'droop' as the 'centrifugal torque' of the eccentric clubhead takes hold. This will bring the CofG axis closer to the shaft axis because the clubhead is flexing toe-down.
At Impact, the full effect of the eccentric CofG on the shaft tip occurs ... droop and axial torque. This is the context that I start with trying to understand your comments on plane, plane angle and CG of the clubhead. Do you see it in the same manner?
On plane to me is simply from the start to the finish is there one single plane which the club is traveling on. It doesn't matter if it's higher or lower than the backswing plane, just as long as from the top of the backswing to impact and on past impact is it going on one plane. If it does not travel on one plane then that means some sort of force has had to have been applied to redirect the club back to the plane by impact. Thus, getting under plane requires a movement of some sorts to try and lift the club back up to the plane.
For me, my club goes steep to start the forward swing. A lot of that has to do with my very big pivot in the forward swing. I have to get my hands down fast in order to catch them up to my shoulder turn so that I'm not wide open at impact.
So, my steep pull of the club vertically down causes the shaft to be vertical at the beginning of my forward swing. Unfortunately that means the momentum of the club is such that if I were to continue pulling the club in this direction it would strike the ground somewhere between my feet and the ball. Therefore I have to LIFT it back up the plane.
By the time my swing reaches the delivery position, the clubhead is way behind me and the butt end of the club is pointing out to the right. This happened because my hands are starting to get thrown out away from me and towards the ball while the clubhead is still trying to go "down" vertically. At this point my only hope is that I have just the right amount of throwing my hands out to redirect the clubhead back to the ball.
The side effect of this is that my hands will be quite high at impact which puts the clubhead in a toe down position and a path that is inside to out. Pushes and push fades are my normal shot because of this.
By having a flatter look to the shaft at the start of my forward swing I am essentially moving the club on a single plane toward the ball right from the get go. I no longer need to lift my hands up at impact just to get the clubhead out to the ball. That makes my hands stay a little lower at impact and using the lie angle of the club properly. No more push fades.
Right now I'm fighting pulls and pull draws now after this fix which is a godsend for me. You would expect to see someone pulling and pull drawing the ball after fighting so hard to shut the face down enough to get rid of the fades. My old release of the club closes the face early so that's why now I'm fighting the pull. I can finally release the club more up the plane than rolled over and shut like I used to have to do.
It also means I can use more of my big pivot to my advantage. I can let my body turn more without worrying about whether or not my hands are catching up.
