What should be the relative positions of a wing's aerodynamic center and center of gravity?
1) It doesn't matter much. AC is relevant mostly for aircraft stability, so it is its relationship to the CG of the whole aircraft that matters. Only when the wing is the aircraft (flying wing), the distinction disappears.

Mass distribution (and thus CG) of the wing matters for aeroelastic effects, but then AC is not very relevant.

2) There is little sense in trying to determine spanwise location of AC. AC is related to the lift changes over angle of attack (AoA). When AoA changes, lift (CP) moves forwards and backwards but not sideways. Theoretically, if you consider a half-wing, you might be able to determine spanwise lift movement, esp. on a swept wing. But again, for stability analysis (where AC is relevant), you never need to do that, so you consider the whole wing and relate everything to its mean aerodynamic chord (MAC). L/2, by the way, happens to be on the airplane's plane of symmetry (L is normally the whole wingspan).