C: Protestants do the same thing, all the time. Study up on the history of premillenial dispensationalism sometime.
Actually, I agree with you on dispensationalism, but it does not make Protestantism in general accretionist, because Protestant churches have not set up mechanisms as the RCC and the Mormons have (the papacy and the president of the council of the twelve, respectively) to provide additional (or revised) dogmas for their respective churches.
But I do agree that Bible-faithful churches must be continually sensitive to the possibility that their mutual efforts at systematic theology (purportedly drawn from the Bible) can nonetheless amount to 'accretions by common consent.' Placing dispensational theories and 5-point Calvinist theology on a par with the Bible itself are good examples of this problem in Protestantism.