Good point and this has to do with physics relative to impacts.
The best way to demonstrate accute impact loading v. prolonged loading is to take a straw pinched between two fingertips held upright (use left pointer and thumb if a righty). Then flick the top of the straw. Fast enough and it'll only bend up top immediately beneath the point of contact. Then push on the top of the straw and it'll bend on the bottom due to the moment arm down the straw (from a prolonged load). This display of physics is represented by the upper inboard side of my seat being slightly tweaked forward (by an accute impact from rear seat's right shoulder) and the lower outboard side of my seat being bent forward (by my upper body's prolonged load thru integrated seatbelt). If the rear passenger's load was more directed against the back of my seat (ie. if he didn't fly up the middle to the dash), then both the outboard AND inboard bottoms would exhibit similar failure. But the bottom of the inboard side was not bent (or at least nowhere near as much as the inboard top corner and outboard bottom side). The rear seat passenger's velocity towards the dash was unbelievable. Knowing the airbags wouldn't timely deploy and I was in the seat with the least chance of survival I actually considered wedging myself between the driver and the steering wheel. Knowing the rear seat passenger was in the middle, the thought of him impacting my midsection stopped me. That's when I rose my left gaurd to absorb impact wondering what the limit of human limbs are. You think this is all impossible to do in such a short period, but when you're facing your death, your mind blossoms with more thought than you ever thought possible. I never once gave thought to whether or not MB's airbags could timely deploy during a high acceleration, low velocity impact before. Never even crossed my mind until that moment. It sure helped having an Engineering degree as the first lesson of Physics 101 is "acceleration is NOT velocity and each results in its own vector that produces two separate forces".
1. There is an obvious picture showing a seat bent the opposite way you state.
2. There is no remaining evidence
3. You clearly state in this passage that actually both bottom sides of the seat were bent, one more than the other. (stands to reason as the belted side would have more force on it handling the combined weight of the unbelted passenger and you.
Your response to this is to discuss a high school experiment with a straw and then complement yourself on your engineering degree for all the options you had during the accident? You must be joking. Do you have ANY pictures on the seat bottom, the seat itself, the outside part of the seat, ect?
Having looked closer at the pictures, Ill tell you what I believe. I believe there is no issue with that seat, that the rear passenger clearly slammed into the seat hard enough to twist the entire upper frame on his way forward.
The impact probably damaged or bent the seat locks, followed by your sustained force, which caused them to fail.
On the driver side, you have an unbelted driver flying forward slamming into everything combined with an airbag deployment somewhere in the carnage of his death.
In back you have an unbelted passenger flying forward, breaking your seat on the way by, and dying against the windshield.
And in the passenger seat, we have you slamming forward against a damaged seat which failed and threw you into the dash before the airbag fully deployed, which then threw you back for your head injury.
If you blame anybody, you should be blaming these idiots who didn't belt themselves in. There are thousands of recorded incidents of rear passengers unbelted killing and maiming belted front passengers.
Im amazed that you would discard the most logical explanation, disregard the effect of a full size adult male impacting the back of your seat, and disregard the obvious results of reckless driving combined with no seat belts.