Thanks for sharing the report with us.
I have comments on two issues:
1. If you look to the right passenger seat where the seatbelt is fixed on top, you see that this side did bend over to the front; also something seems to be broken on the top.
I think it is fair to raise the question, whether this part is stiff enough to withstand all loads coming from a crash.
As a rule of thumbs, it should withstand at least 30g, i.e. the force which corresponds to 30 times of the weight of the passenger.
If it is not stiff enough, it bends to the front, not sufficienty preventing the passenger from hitting the dashbord.
Anyway, Mercedes has a long track record in designing these types of integrated seat belts, i.e. for the Viano Minivan. I have seen those, and they look really VERY stiff and the supports seem to be made of hardest steal.
I would trust those seats more than any other seats!
However, when you say "bent forward by over 40-degrees without a scratch or bruise to my right shoulder" this is an argument that the seat was not stiff enough, I must agree.
2. You complain that the airbags seem to have blown-up too late when you already have touched the dashbord.
Mercedes-Benz is using a step-wise approach in blowing-up the airbags.
If the decceleration is low enough that the belt can prevent you from severe injury, only the belt pretension system is released and not the airbag. Considered you use the belt. If you did not buckle-up, the airbag is deployed immediately.
This has two benefits: First saving the risk of airbag explosion and saving money for repair if not necessary. Second: if a car has first a quite light crash and then a second very heavy one, the life saving deployment of the airbag is not wasted on the first decceleration but can be used for the second heavy one.
So without knowing anything about your accident, could it have been, that there was a quite "long duration" crash into this long limousine? Maybe the first 20 cm of the front of the limousine where so weak (compared to the Mercedes) that the airbag sensor decided NOT to deploy the airbags and only deployd the belt retention system. Then suddenly the decceleration was so strong (because hitting say the engine of the limousine) that then the airbags where deployed.
If this hypothesis would be true (I point out: only a hypthesis), then the system saved your life. Because: If the airbag would have been deployed earlier, it would not be able to prevent you at time of the much more heavy shock in the deccelation process. So pushing you back to the roof could have been the better alternative than having no airbag available (because blown-up and released already a 1/10 of sec before) and fully touching the dashbord with your face.
Of course, this is just a hypthesis to think about.
All the best