OK here's another scenario: I'm on finals to land and think I'm going to overshoot - I stick the nose down let the speed build up to above normal approach speed, I then pull up to regain normal landing speed. Would I still overshoot or because I've effectively gone outside of the best glide speed reduce my range to touchdown?
In theory, yes, but the effect will be minimal, and the large speed excursions are likely to cause more problems than you solve. As cats_five said, you should be varying your circuit - even to the point of S-turning on final.
I don't know the aero theory for the pitch down moment, but the rearward stick movement in a sideslip is always intended to maintain the pitch angle of the glider - the nose is not raised (unless you already let it fall, and are raising it to the same angle). As noted, the ASI doesn't work properly in a full blooded sideslip. In the abscence of power, pitch sets speed; maintain the constant pitch, you should maintain the same airspeed - therefore you don't stall.