The (or at least "a") point of the OP was to show that this is the central difference between the "authority paradigms" of RC and Protestantism. But is this difference - that a viva voce can attempt relative perspicuity compared to its previous statements on a subject - meaningful? Since any clarifications they make are in turn not necessarily perspicuous, then how is the RC any better off than the Protestant? As is said in the OP:
Like the Catholic, the Protestant theologian must use his fallible intellect to locate the source of divine authority. Also like the Catholic, the Protestant theologian must use his fallible intellect to construct clarifying questions regarding the content of divine revelation. But unlike the Catholic, the Protestant theologian must also utilize his fallible intellect to construct clarifying answers to whatever second, third, or fourth order questions must be asked in order to arrive a definition or determination of the content of a revealed doctrine. For in order to clarify or determine the content or scope of some theological matter, such as justification, one must necessarily seek answers to second, third, and fourth order questions as described above.
Given that no Magisterial promulgation is necessarily perspicuous, any answer anyone gives to the question of "what is the criterion by which perspicuity can be identified?" must have been discovered by some other means. And since knowledge and application of this criterion will be a precondition for even understanding what Magisterial proclamations in fact mean, it turns out that the sort of private judgment about which RCs lament follows from Protestantism really follows from RC. Note the statement by Bryan Cross I quoted in a post on private judgment (cf. here):
When we approach Scripture, how do we determine what the Holy Spirit is saying? Either each individual is ultimately his own highest authority regarding what the Holy Spirit is saying, or that authority belongs to something outside the individual. In the former case, we are left with “private judgment,” and the endless fragmentation that must accompany it, as history shows.
I might as well ask:
When we approach Magisterial promulgations, how do we determine what the Magisterium is saying? Either each individual is ultimately his own highest authority regarding what the Magisterium is saying, or that authority belongs to something outside the individual. In the former case, we are left with "private judgment," and the endless fragmentation that must accompany it, as history shows.
After all, how many popes are there again? How many authoritative [and extra-biblical] traditions? I dealt with Cross' statement on Protestant terms in the other post. Here, the point is simply that the tu quoque response those at CtC spend so much time trying to refute is shown, right here, to be valid. The tu quoque against RC is not that "there is no difference between what Scripture is capable of as compared to a Magisterium." The tu quoque is that the RC paradigm requires private judgment even while it attacks Protestantism for allegedly requiring it.
Ironically, the real difference between RC and Protestantism is something about which both groups can agree: in comparing RC and Protestantism with their sources of authority, only Protestants believe their source of authority is necessarily perspicuous. Rather than demonstrating RC to be necessary, the CtC post has once again shown off Protestantism as the more attractive option (cf. here).
As an aside, the author of the OP said the criterion of perspicuity must be objective without ever actually explaining what it is (comment #50).