What's missing is inertia. If the ball was dragged along the swing path during the compression phase then this must be taken into account.
I'll try to put down my analysis of the situation from a pure physics angle.
(If you're not interested go now and play some golf ).
I'll use the situation in Club's picture (open club face to swing path at separation).
Let's look at the movement of the ball relative to space at the instance just before of separation. It was:
- dragged along the swing path by the club head,
- dragged by the turning club head in the direction perpendicular to the club face.
- moved by the decompressing club face, which I assume to be decompressing perpendicular to the direction perpendicular to the club face.
#1. Combining the above, the direction of the movement of the ball just before separation is between the direction of the swing path and the direction perpendicular to the club face.
#2. The club head was turning during the compression face and this also drags the ball to be turning (relative to space) as well.
For this analysis I ignore the situation when the ball actually rolls up the club face during the compression phase.
Now look at the forces pushing the ball at separation.
If we assume at separation the club face is flat again (no compression and no bulge - not sure if this is true) then Club's picture accurately depicts the situation. The force perpendicular to the club face is a combination of the force from the movement of the club face *and* the spring of the club face back from compression. The force paralell to the club face should be only from the movement of the club face as I assume the club face decompresses in the direction perpendicular to the club face. Regardless, if the clubface is flat at separation there are two forces - one perpendicular to the club face and the other parallel to the club face. (Of course to the ball there is only one force but we break it up into two forces for analysis as they affect the ball differently).
So: At separation the ball is
#3. pushed by a force perpendicular to the club face,
#4. pushed by another force parallel to the club face towards the bottom of Club's picture.
4a. Since the surface of the ball is tangential to the club face and there is friction between the surface of the ball and the club face, force #4 spins the ball.
4b. Precisely because of friction, force #4 also pushes the ball (but probably only a little bit) "down" along the direction parallel the club face.
OK, finally the combination of the above.
1, 3, 4b - at separation the ball moves in a direction between the swing path and the direction perpendicular to the club face. (Ha, ha, what's new?) Is this direction closer to the swing path or the direction perpendicular to the club head? This is affected by a lot of factors as is evident from the above analysis. Different factors may dominate in different situations.
2, 4a - the ball spins in the direction as depicted in Club's picture. This causes the ball to curve right at some point.
OK. Enough physics. I'll go hit some balls now.