...that the suggested formation of the flinch response is palms down and raised to chudan position covering the throat and chin. This results in the exposure of the inside of the forearms and negates the protection afforded the center line. I see this as problematic for a system that purports to provide an initial defense against a surprise attack that can manage empty hands to a veiled armed attack.
I don't see a problem at all since the 'spear' is ONLY the initial movement as a response to AN attack, rather than a posture within which one receives attacks.
If it were the latter then yes, there'd be a problem regarding knives. But it isn't.
I reckon the 'spear' wouldn't work with the other side of the arms : the structure would be weaker and the hands couldn't do so much in the moment after application.