How can wingmounted engines allow for lighter wing structure.
Torsion damping can not possibly be that heavy on a modern wing.
I think you misunderstand. With no engines on the wing the structure must be torsionally stiff, and that takes material to do (the torsion loads are usually reacted by the skins which must be thicker to take them). But if you have nice lumps of metal on lever-arms you don't NEED to make the wing torsionally stiff - you just give it a high rotary inertia so that the time-constant on reacting to a torsion upset to likely to be so long that the upset will disappear before it moves significantly. Pylon-mounted engines do just that; they act as torsion-mass dampers, so the skins don't need to be as thick.
Compared to all the crap that is associated with an engine on a lifting surface.
Heavy engine on wing = more wing and heavy wing.
I'm sorry, but that's not true. I'm also sure it has been detailed here and elsewhere many times before - google something like wing bending moment relief. Essentially if you put all the weight in the fuselage and all the lift in the wings then all the weight is reacted at the wing root in bending. But if you spread weight across the wings the weight pushing down balances the lift pushing up and that part of the lift never has to appear in the bending moments (either summed at the root or reacted progressively across the span) so the required wing bending strength is much smaller. Therefore you can get away with a much lighter structure.
That's the argument for wing-mounted engines, undercarriages, wing fuel tanks, tip tanks, military stores mounted on wing pylons etc etc.
All of this has been known about since the 707.