Just a few thoughts.
Helicopters at speed at low level do leave a wake on the surface. Wessex over Irish Loughs definitely do. Yes it is more messy than that of a fixed wing machine due to all the mixing vorticeys, but it is there.
The analogy of a rotor disc to a solid disc is of course not 100% correct as air passes through a rotor disc! But in many respects the overall result is not too dissimilar, hence it is a useful visualising idea. Pulling the rotor disc back with cyclic is similar to pulling an aeroplane's stick back in that lift and drag increase resulting in a climb and deceleration.
Lift produced by a rotor during auto is the same as in the hover! That is equal the helo's weight. One of Newton's laws about a body remaining at rest or constant linear motion unless a force is applied to it. ie a steady state auto all forces balance out- lift=weight, same as in the hover.
Ground effect comes from the fact that air will not pass through the surface. When any heavier than air machine flies it tries to throw air downwards, the ground stops this and produces a higher pressure below the aircraft. The closer you come to the ground the more noticeable this effect becomes. In helos this "back pressure" reduces the induced flow and hence brings the blades lift vector nearer the vertical reducing drag. Induced drag is significant. In fixed wing bigger aspect ratios reduce induced drag, because this effect is significant most aeroplanes have relative high aspect ratios. If you could hover with your blades virtually touching the ground your power required would be similar to that needed to keep your blades spinning at zero pitch on the ground, as there is no lift induced drag. Unfortunately helicopters have their blades on top so you will not be able to prove this for yourself, but the physics is right.
Despite all this, back to original question the effect during an auto......negligible.