This is the way it reads to me.
For normal combat, choosing to parry is a conscious decision.
You can't choose to parry in Frenzy.
I also think the crit takes precedence, so "Must Parry", means that the opponent must parry. You don't have a choice, you are just made to do it in combat. Now you still can't allocate anything to Parry, since you are in Frenzy. So you must make a +0 Parry roll. You didn't mean to parry, you were forced to, or did it by accident. This means you can still fumble your roll, just like normal, (but you get no OB to add to DB for the parry, whereas if you weren't under "Must Parry" and weren't under Frenzy, then you could choose to add to DB - the penalty for the Stun/Must Parry).
If you get hit while under Frenzy, then you could choose to rule that you get a bonus to leave frenzy, or a penalty to stay in frenzy, which is the same thing, depending on how you read it. Frenzy is a Self Control skill so you could handle it like Meditation. While meditating, a hard strike gives a +25 to leave meditation (or a -25 to stay in Meditation, it's all how you read it).
Just a thought. YGMV