G,
I disagree with your statement: "Va can be considered constant regardless of aircraft weight, since the structural strength of the aircraft is actually based upon N1 at MTOW, and not just N1 in isolation". It is true that structural strength is a function of N1 x W, but there is inevitably an overriding maximum value of N1 which must be respected irrespective of AUW.
P,
I suspect that the different values of Va at different AUWs are based on a constant N1 limit. I agree with G that it is
Vb that is important for the flight phase that you describe. Remember that Va is relevant to full deflection control inputs and the stall, not g limit exceedances in turbulence.