No problems disagreeing Hipennine - that's what it's all about - however, with great respect to your Ergonomics pedigree I feel that the upper body comments somewhat miss the point.
Some kind of compromise upper body brace position has to be established that is practical for a majority of passengers at the normal economy seat pitch range of 28" to 32". It is possible (I don't know) that the position may increase the probability of a broken neck, but that has to be measured against the (demonstrated) probability of unconsciousness from head impact.
There has been a considerable volume of work completed on the Brace position and notwithstanding your credentials, the "authorities" have come up with a viable position, that is perceived by the industry to be realistic. The danger in having individuals come up with possible well-intentioned variations is that they may confuse others. You may well be correct about the inherent dangers of the upper-body brace position, I cannot tell: However, until a majority of your peers agree and the Brace position is adjusted accordingly, please excuse some scepticism.