Re: Does Extension Mean Straight? or Central Axis Neither Central Nor Axis?
My thought is that, per your first examples, the cause of your shoulders being pushed back and your body collapsing backwards at the lower back is the result of tension/muscles firing in those areas and not the overall posture of your body. The same should hold true for the other postures, as you're depending on muscles to hold those postures for you.
If you completely relax the body, shoulders down, core relaxed, chest relaxed and lower back relaxed, you'll feel the front of the body kind of collapse down on itself and in, this naturally brings the spine into a position more like your final example, but again, natural and relaxed vs. being held in that position by muscles. Doing so should result in the forces in your diagrams pushing or compressing the entire body down, rather than breaking it the points indicated.