The only way I know of to get a side force from the fin, rudder central, is with sideslip.
No one is suggesting that the rudder is central for zero sideslip. It may be a little less than for the wings level case, but there's still substantial force coming from the vertical tail surface (without sideslip).
All of the HORIZONTAL forces involved can be simplified, I hope you will agree, into thrust component, drag, horizontal lift component and the one that most of you want to ignore which is the horizontal force generated by sideslip.
And you want to ignore the force at the vertical tail...
2. Horizontal lift component X arm to ?? Whoops - where is the opposing force.
At the vertical tail...
Certainly you can push it sideways a bit by having an unbalanced transverse load. But so what with our present thoughts?
I sympathise with your concern that unquestioningly resolving everything about the C of G is unhelpful. However, in the case of the horizontal component of lift, there's a reason for considering the C of G. The total (mainplane + horizontal tail) lift on the aircraft must act at the C of G, otherwise there would be an unbalanced couple with the weight. Thus the horizontal component of lift also acts at the C of G and the C of G position is therefore of significance in assessing asymmetric control.