This doesn't make much sense to me. What's so important about the torpedo bulkhead? We know that heavy shells penetrated and exploded in the engineering spaces:
This requires a penetration of the belt or the main armour deck. You state that the main armour deck was just below the waterline, which would be a problem, as perforation of the upper belt would allow flooding to spread out across the top of the main armour deck and to enter the turbine and boiler rooms via the shell holes.
Actually, http://www.kbismarck.com/proteccioni.html states that the main armour deck was actually about a meter above the waterline, but maybe the combination of flooding from POW's hits, counterflooding to correct the list, subsequent damage in the final battle and the heavy seas had reduce that meter of clearance to the extent that flooding could occur freely through the heavy shell holes into the citadel. In any case, it seems clear that gun penetration of the torpedo bulkhead isn't important. Of course, this doesn't tell us anything of the significance of scuttling attempts, but it does suggest that gunfire had significantly decreased Bismarck's buoyancy and stability.
It should also be noted that HMS Rodney did close in on Bismarck to some 2000 yards only, fireing at this point blanc range full broadsides into her target. As the 16 inch shell was fired from such a close range, it would hit Bismarck's side at almost straight angles, as the guns of the British ship did not even need to elevate. Even if Bismarck had her sides protected by 30 inch armor, this would not have been enough. At this close proximity the AP shell's used would penetrate any armor.