Hi TryingToLearn... I think you have a good understanding of the workings of the door plug, especially the bit about the Coefficient of Friction of metal on metal being 0.2. I had originally thought that the pads were rubber covered, so giving a CoF of 1.0. until I saw the close up picture of the plug laying on the ground.
Maybe the ultimate design fault is that as the differential pressure rises, the plug moves outwards, away from the support frame. In most of the doors used, they are on the inside of the door frame, so any outward movement increases the sealing force.