I understand the above is on flight load limits, but can we assume that even passing Vne we still have some safety margin ?
NO!!! Absolutely not.
Wing bending moment and load factors are not the primary criteria in determining Vne. Go back and read about flutter again.
There is one more speed after Vne, Vd design speed that is where you are guaranteed that the wings will come off in a dive without any control input at all in still air conditions.
No, absolutely not.
Vd, or Design Dive Speed, is a reference speed, and it's not a cockpit speed you'll read on your airspeed indicator. It's provided as Equivilent airspeed, and it's also not something you'll likely find in the AFM/POH for your airplane. It is NOT the speed at which the wings come off.
Your limitation with respect to maximum speed is Vne. The wings don't separate there either, unless you pull them off, but you still need to religiously respect the limitations for the airplane. Design diving speed is not a limitation. Vne is. Design diving speed is a reference speed upon which certain tests and other criteria is based.
Additionally, the word "design" in the various certification speed terms does not mean that is what the airplane was designed to do, but rather that it's a reference speed for certain certification criteria, for that airplane design, or airframe.
If your aeroplane has a design load factor of 3.8g in one axis, when carrying out a rolling manouvre (e.g. two axis.....a botched barrel roll for example) this HALVES to 1.7g !!!
That's a good guess, but no...it doesn't decrease by any particular percent. There are far too many variables involved. You're correct, however, that additional forces beyond simple loading are involved, as the airplane is also under torsion (twisting), bending, tension, compression, and even shear loads in any maneuver. The interaction of these forces compounds and becomes quite complex in the presence of multiple control inputs and rapidly changing combinations of forces. Even a single control axis with inputs in both directions has already exceded the boundaries established by maneuvering speed...Va. Once you reverse the direction of the controls you've already gone beyond that upon which Va is predicated; a single control input. Compound that further with multiple inputs of the same, or different controls, or any combination thereof, and you're no longer in the same theatre as the stage on which Va was set.