I agree... the arrow didn't do it. But, I think the film editors wants us to believe that.
The fact that you know all that, I find very impressive. just a matter of anatomy and wound characteristics... of course when you add in the human factor, anything's possible...
A bodkin tipped arrow would not slice a horn like that... I'm thinkin' sword stroke (obvious alternative)... there was that kind of odd bullwhip sound during Boromir's dispatching of the last Uruk... anyone else catch that?...
Allow me one comment on Legolas' skills... while I am not questioning the accuracy or speed portrayed, I was just a little bothered by the effects of his arrows... an arrow is not a bullet, and possesses no where near the immediate lethality... an arrow kills through hemorrhage, and a broadhead is geared toward causing maximum blood loss quickly... Even with a heart shot, double lung hit or a hit that severs a major artery, a mature deer still has about 30 secondsto live and major adrenaline to push him...
the fact that he's using bodkin tipped arrows means his targets would die even more slowly...While a head or a heartshot would stop a man (orc) in his tracks due to shock , a hit anywhere else but the spine would not drop a man, much less a Uruk (IMO) in his tracks... kill him, yes... eventually...
the scene of Legolas' killing spree makes my point nicely... a recurve bow does not possess the energy to knock a full grown man back onto the ground, much less go thru two armored Uruks at once... everything in me that I know about bows and wounds comes from experience as a hunter, EMT, and a Marine...
It looks way cool on screen though...no question about that...
Boromir's death was completely believable, considering the arrows and shot placement... he would have time to fight on, because the bodkins don't cause the massive blood loss... had the last arrow been 4 inches to the right, skewering the liver, he would have gone into shock...and we wouldn't have heard any of the "my captain, my king" speech...
The immediate lethality of an axe, no one can question... I mean, when a major portion of yer internal organs are on the outside of yer body, that's gonna ruin yer medical record...