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.
Legitimate concerns and something for me to think about... But if I am truly relaxed in all those areas, and not just squishing into a bag of bones of the floor, then something is holding me up.
So the movement organization of holding me up could be done in such a way as to deform me into those two middle diagrams I have above if I was holding the "wrong" way... Or it could, as you indicate, collapse me into a more ideal structure, but only if I have degrees of freedom in the right places and to the right extents by organizing structure in other places.
So still you get, where is that structure and what's it doing that allows those places to be relaxed and not actually need to carry the stress in the first place? I guess what I showed was an example of what I think is one such pathway, though here it is active rather than passive.