shoulder harnesses were instructed to be level (perpendicular to spine),

Can't be both. Level to shoulders would put the mounting point much higher than perpendicular to spine since seats are always tilted back. If mounted perpendicular to spine, and lap belts were cinched down properly, in the event of an abrupt stop, the torso would act as a pivot point, head and shoulders trying to go forward. There would be no compression force on the spine since that would be a perpendicular motion to the straps.