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[slats11]

Hard to give anything more than a semi-quantitative answer to this.

Compression fractures of the lumbar spine are relatively frequent in pilots ejecting from fighter jets. Typical forces involved here are in the order of 12 - 14G. The bony pelvis would be expected to withstand these forces. Note however that:
a) we are talking about young adult males (20-30's) where bone strength is at a peak
b) these pilots are wearing a harness
c) the seats are engineered to deliver this vertical acceleration as safely as possible.

Very different in a commercial airliner - heterogeneous population, lap belts only, in seats not designed for this vertical acceleration. So you certainly would start seeing some compression fractures at much lower forces.

If you were talking about a fall and landing in the sitting position, it would take a fall from at least a couple of metres onto hard ground or concrete to cause vertebral fractures plus pelvic disruption in a young adult male (and obviously a significantly higher fall if landing on legs as decelleration will be more gradual). An elderly person can suffer the same injuries with a fall from the standing position.

Not sure that this helps much however. Without knowing over what distance the vertical speed was reduced to zero, you can't really estimate vertical velocity even if you can take a good guess at the peak acceleration.
v squared = 2as, so you need a good estimate of s and a in order to even guess v.

Also, it is almost certain that a was NOT constant - injury pattern (sort of) correlates with peak a rather than average a.