John, I suspect that we are actually saying the same thing.
Whenever the lateral forces are unbalanced the aircraft will accelerate towards or away from the live engine. So whenever the aircraft is in a steady velocity sideslip, these forces must be balanced.
In your case (a) with wings level, the aircraft will be slipping towards the dead engine but the ball will be central. My slightly modified version of this is when the nose is then yawed towards the live engine just enough to make the direction of the slip to be down the original track. Once again the aircraft will be slipping but the ball will be central. If we interpret the position of the ball as an indication of slip (as in the original question in this string) then the ball appears to be telling lies in both cases.
You case (b) with slight bank towards the live engine uses a lateral component of lift to balance rudder force, such that lateral forces (relative to the earth) are in balance. This time there is no slip but the ball is out towards the live engine. Once again if we interpret the ball as indicating slip, we find that the indication is incorrect.
I agree that the ball should not really be interpreted as an indication of slip, but the fact remains that this is exactly how far too many pilot use it. The JAR questions ask for the ball position and not whether the indications are true are false. But almost without exception students initially get these questions wrong, because they consider the ball to be a slip indicator.