I think I start to understand
The moment exists physically, which I was not sure. I mean, if the wing was made of paper or something frail it would be bent and maybe even folded or broken because of the couple. And its effect is all included in the cp.
Now, regarding the tail lift as being negative or positive, I had the notion that it had to be always negative because of the aerodynamic moment, and that a tailplane was necessary (an then the germans invented the flying wing)
But what I think is just that for the airplane to be in trim, the tailplane lift has to able to balance all the other pitching moments, whichever they are. These are basically those due to the wing plus fuselage lift at the wing-fuselage cp, the thrust and the drag.
The Drag-Thrust couple usually gives a nose up (positive?) moment, and the Lift gives a nose down (negative?). The tail lift has to balance the resultant of those moments. I guess that the Lift effect is much greater than the T-D couple? It seems so if Lift is many times bigger than Trust for Drag, but it depends on where the wing-fuselage cp lies. I deem it does lie well aft of the CG?. That would be why the tailplane has to produce a pitch up (positive?) moment with negative lift. But can the cp be ahead of the CG for "normal" angles of attack?