They are claiming the rear-seat passenger was the only force that bent my seat back forward now. Mercedes is also claiming their airbags worked accordingly with a steering wheel pushed into the dashboard and two front seat occupants with documented top/rear head injuries. If that's what "performing under extreme conditions" means to Mercedes, they just lost all credibility.
Well, short comments:
1. Why shall a rear seat passenger be able to bend the seat forward?
The seatbeld, as a very rough rule of thumb, should withstand 30 g (I do not know whether this is in any law or spec, but I think it is reasonable. For aircraft, by the way, the max load is lower).
Imagine, your weight is 160 lbs. Than the integrated seatbelt-system should withstand the force equivalent to 160 lbs times 30. (which is appr. 24,000 Newtons (to use the correct unit for force).
Imagine further on, your weight is 320 lbs, than it should withstand 48,000 Newtons. And I think, there are passengers weighting 320 lbs; and an integrated seat belt system should be save for them, too.
And now, considered your weight 160 lbs and the weight of the rear seat passenger is also 160 lbs, in total this comes also to 320 lbs.
What does this mean: If the right site (where the seat belt is fixed) of the seat withstands an 320 lbs passenger, it should also withstand an 160 lbs passenger plus the load of the 160 lbs of the rear passenger.
I.e., I cannot understand while a correct right site of the seat should be bent by the impact of the rear passenger.
2. Also consider, if there is a severe impact into the driver's airbag, I also can imagine that the steering wheel ist pushed into the dashbord. The airbag is only supported by the steering column. And if there is force on the airbag, the column might also be pushed into the dashbord.
Another comment: You did say the crash did take appr. 1 sec. I Cannot believe this, I think this time is much too long.