I really don't get the confusion here: if the critical says "Stunned" or "Must Parry with % of OB" then the individual cannot attack* - SO NO ROLL. If you are not attacking, you are not rolling the dice. There is no rolling for parrying - you just shift the points over to your DB and done. I have been playing RM, off and on, since it first came out, and I have never - never - rolled an attack/fumble check when it said that I COULD NOT ATTACK. <------- And no one I have ever played with has brought this up; this is completely new to me.
Now, if does say that you can attack, then you can attack, and you can then fumble said attack.
*It even says it here: RMC Arms Law, pg. 49.