It's late at night that I'm reading this, so I won't commit to any of the mathematics other than to say that your reasoning sounds good.
I see a few problems and I see one solution.
Problems: In order to work in "real life" you would need the devices to make contact with whatever the vehicle was colliding with. But say for example, a vehicle collided with only the right rear of the vehicle ahead. This would diminish contact points and reduce the mitigating action.
For rigid compression as you describe, effectiveness requires straight line collision. Shear angles would have the effect of crushing the device, perhaps before it can absorb the impact: think of the difference between stabbing with a pencil directly or at an angle to its length.
Solutions: Semispherical shapes might handle shear stress the best.
And one other solution: Think heart chambers. Blow the compressing material (air, fluid, whatever) through a one-way valve into the force-easing chamber which will widen laterally instead of forwards and back. The valve will prevent rebound. Something like this should be embeddable in your design, even a series of such chambers in miniature.
Since I am unable to contribute anything beyond this, I will opt-out after this comment. But if something I said is really useful, you can always come back to this and accept it. :-) Good luck to your, Ed; I hope you succeed.