Why Should the Zohar be Considered a Halachic Source?

I recently wrote a paper that addressed the question of whether the halachah can follow the Zohar when it contradicts the halachah as determined by the poskim. My basic thesis was that it depends on who wrote the Zohar: The earlier the Zohar's author lived, then the more authoritative it would be.

In truth, though I couldn't find the source, this isn't my own idea. It comes from a quote from somewhere or other to the effect that: כדאי רשב"י לסמוך עליו, Rabbi Shimon bar Yochai is worthy to rely upon. This is a clever argument because it means that if you don't follow the Zohar, you are denying Rashbi's greatness.

However, there is a serious problem with this whole idea, and that is: Why? In the klalei halachah, it is a rule that יחיד ורבים הלכה כרבים, majority rules no matter who the minority is. The best illustration of this, I think, is the machlokes somewhere in Bava Metzia regarding some obscure law of tzoraas. If I remember correctly, an Amora met Eliyahu HaNavi, who said that the Heavenly Yeshivah was studying the same law, and HaShem held one way while the Academy held the other. Unsurprisingly, the halacha follows the Academy, not HaShem. The point of the story is clear: appeals to authority aren't going to help you. So what difference does it make who wrote the Zohar? He's at best a daas yachid?*

The problem is compounded by the GRA's statement that the Zohar never, ever, contradicts the Gemara. Whenever we see such a contradiction, it is either due to a misunderstanding of the halachah or the Zohar. If Rashbi wrote the Zohar, how can we say that? The Gemara clearly does not follow Rashbi all the time!

Another question is that regardless of who wrote the Zohar, if we want to give it quasi-Talmudic stature, it needs to gain it. How would that happen? How did the Talmud get to be halachically binding? According to the Rambam's introduction to Mishneh Torah, this happened because all the Jews accepted the Talmud as binding. Its halachic authority only exists because everybody gave it that authority. But in the case of the Zohar, clearly there were many communities which did not give the Zohar this authority, even if they in theory believed that the Zohar was "genuine" in some sense. A good example would be the Yekkes; despite the fact that Rabbi Samason Raphael Hirsch did respect the Zohar, he did not say "Berich Shemei." (This is an interesting contrast to Prof. Yeshayahu Leibovitz, who had nothng but contempt for the Zohar but did say "Berich Shemei.")

So why does anyone at all consider the Zohar a force to be reckoned with?
*I've noticed that there is no punctuation for the yeshivah "Ay...kashye" construct. Maybe I should use the ArtScrollian "-?-".