Is A380's center of gravity really too far back?
No. Positive lift on the tail does not necessarily mean that the center of gravity needs to be aft of the aerodynamic center of the aircraft. Only when the lift per area of the tail surface is higher than that of the wing will the center of gravity move into the unstable region.


  the center of gravity is always closer to the nose of the aircraft
  than the aerodynamic forces resultant


Again, no. The center of gravity needs to be at the same lengthwise station as the center of pressure in trimmed flight, otherwise the aircraft would not be trimmed and start to pitch. With low-mounted engines there might be some thrust moment which must be canceled by a slightly forward location of the center of gravity, but this effect is small.

Next, the auto-stabilisation is not only helpful when stalling. It also provides a smoother ride in gusty conditions and makes it easier for the pilot to fly the aircraft.

Since the aspect ratio of the tail surfaces is smaller than that of the wings, lift there is bought more expensively, so it will be bad for performance if the tail produces more lift per area than the wing. Negative stability is only helpful for supersonic and highly manoeuverable airplanes. Airbus does not do that, and besides, the A380 would not be certified with negative stability.

What it does have, however, like all Airbus aircraft since the A310, is relaxed static stability. The center of gravity is still ahead of the neutral point, but less so than in conventional airliners. This does indeed help to lower drag, and the reduced (but still positive!) stability is enhanced by electronic means.