If you are downed, pinned, and subject to leg-breakage at will, I'd say you're in no position to attack, regardless of other considerations. I recommend begging for mercy, offering service or bribes if spared, or praying for divine intervention. In any case, the definition for no parry in my Arms Law (RMSS version) limits allowable actions to maneuvering (at a minimum of -75), so no attacks. Without the stun result, attackers don't get the stunned bonus. And, yes, stun removal effects won't help. Otherwise no difference.
If the attacker chose to break those legs, I think I'd throw in some rounds of stun as well, though.
I agree with yammahoper on the must parry interpretation, except that the +0 attack should always be rolled. No defense without the risk of fumble, weapon breakage, or accidentally stabbing someone you want to take alive.
Not sure where yammahope gets that extra +30 against the stun no parry foe. That'd be pretty excessive considering he can't parry and you can go full OB. You get another +30 if the target is downed, but there's nothing about no parry that implies that.