I strongly agree that the chance to continue an attack while being hit should be largely a product of weapon weight and timing of the attack. Once a bardiche, greate axe, or maul gets going, like someone else said you couldn't stop if it you TRIED.
I study a system of Japanese martial arts which dates back to their major civil war period. Some of their larger battle axes, warhammers, and polearms, were "swung" much like you would perform throwing techniques on another person. You simply could not just pick it up and swing it around with your shoulder and arm muscles like the M&B characters are animated doing, its more like using your body as a fulcrum and the weapon shaft as a lever, and casting it into an enemy with combined leg and arm movement. Although European martial arts are not as well preserved as the Japanese, I've heard that the methods of using great swords and other huge melee weapons were quite similar (simply because they are human beings following the laws of physics
).
On the other hand, getting nailed by a huge warhammer is going to move you and definately disrupt your line of attack. Perhaps both the attacker's and interrupter's weapon momentums should be the deciding factor (momentum persumably being the weapon weight and time into attack).
All this aside, I think that the problems with getting mauled by mobs of opponents in a melee would be partially fixed if attacks are stopped by friendly troops. Friendly fire for melee weapons would get greatly out of hand without some major fixes in AI (though it would be a very interesting addition). But really, it should be VERY difficult for people to slash at each other in a crowded space. There's a reason most armies use spears
Speaking of, it seems like a relatively simple feature to have the AI try to maintain a distance from his opponent equal to the reach of his weapon, and damage should be reduced the closer to the enemy you are than this. Between that and "friendly stoppage," crowded melees should be much more manageable.