"Finally, if full flap has been used during the glide approach on final and it becomes apparent an undershoot is occurring then clearly you are in potentially serious trouble"
On a glide approach flap is used to bring the aiming point towards you therefore, it should not be selected untill you are guaranteed to make the aiming point without it! i.e. at 400 feet on Final.
The whole point of practicing the glide is to learn to assess the vertical angle between yourself and the aiming point and then to maintain that angle. Once established on final you can use flap to steepen the approach and bring the aiming point towards the aircraft. f you can't achieve that, then practicing long straight in glide approaches is both a wate of time and at worst likely to teach low houred pilots an inappropriate technique.