Beyond RPL!
Why Vs moves with CofG.
Weight act in front of lift producing nose down moment, the tailplane counteracts this with a downward force. If the C of G is further aft there is less nose down and therefore less downward force requred from the tailplane. You could say the aircraft is effectively lighter; this will reduce the stalling speed in the same way as a reduction in weight would.
Va, purpose and change of weight.
Structural limitation.
At any given angle of attack the lift generated depends on airspeed, if we suddenly increase angle of attack the force will be greater at a higher airspeed.
A heavier aircraft will take a greater force to move than a lighter one.
Therefore if you take two identical aircraft, one at MTOW and one with Pilot and minimum fuel flying at the same speed and suddenly increase the angle of attack to the critical angle of attack they will both generate the same force, but the lighter one will have a greater vertical acceleration due to its lower mass.