Cleeds, The fact that you cannot see the forest for the trees does not make me wrong. Of course headshell offset angle and other reasons for lack of tangency of the cantilever to the groove are what results in the skating force but there would be no FORCE, with a capital F, without the need to oppose the force of friction between stylus tip and groove. Can you get that?
Have you ever studied Newtonian mechanics? Sit down with a piece of paper and draw some vectors depicting the pull of the stylus that is due to friction in the groove and the need for the tonearm, cartridge, cantilever, and stylus to oppose that force, which is ultimately expressed as the skating force, pulling the cartridge toward the spindle. This happens because the tonearm is stiff and the connection between it and the cartridge is solid. Otherwise the actual force that counteracts friction is toward the rear and inside the arm wand, but the cartridge can’t move that way because of the stiffness of the structure, so another vector is generated which ends up in the skating force.
As I think you have understood, in a straight line tonearm the force of friction is in a straight line with the pivot, if the tonearm is ideal which doesn’t really happen. But in that ideal case there is no skating force because there is no angle between the force of friction in the groove and the force needed to counteract that friction to prevent the cartridge from flying off into space.