IMHO, this stuff about seat belts and broken bodies sliding out is all conjecture and probably irrelevant.
What is relevant is that the seats sink - occupied or not. The seat cushion alone can float. But the seat as a whole sinks. And even if subsurface currents cause the bodies to slide out (and I guess this is conceivable), by that time the body too has sunk. And will stay sunk.
So this scenario can really only happen if the bodies slide out quickly due to wave action in the initial period when the seat is on the surface ie before it gets waterlogged and starts to sink. Look I guess its probably possible. But I can't help thinking that unrestrained pax are the more likely explanation. Especially when we take into account the lack of Mayday and the absence of life jackets.
We don't know exactly what injuries the bodies displayed. We have vague and likely incomplete information about a proportion of the bodies. But we don't know the full picture. I do understand the honorable desire to spare relatives the details. But the fact is we can't make too many conclusions on the results released..
People who die jumping off buildings routinely experience pelvic fractures and spinal fractures. You don't have to be strapped into a seat.