My response to a FaceBook friend who asked for my understanding on the authorship of the Zohar:
Dear ... ,
You pose an excellent question.
I hope I am not disappointing you, but, truthfully, on the logical surface level I have found the academics' arguments more compelling. They point to things like language style and that the Zohar did not seem to be written by someone who used Aramaic as his first language. Rabbi Aryeh Kaplan, in "Meditation and Kabbalah" tries to make an argument in favor that it was literally written by Rabbi Shimon bar Yochai. However, he admits that the relevant pages from Rabbi Isaac of Acco's investigation into the matter are missing from the manuscript. He accepts Rabbi Isaac's conclusion without the benefit of being able to examine how he came to that conclusion.
Therefore, I think that it's likely that either:
(A) The Zohar, being part of the oral tradition, came to the Spanish Kabbalists in oral form. They merely gave it, it's written format, with possibly a bit of embellishment. This would be no different than Ravina, Rav Ashi and later Rabbanim Savurai giving the Talmud it's basic current written format.
(B) They channeled the saintly figures of the Zohar. This is not entirely unheard of. For example, Rabbi Chaim Vital speaks of learning Torah from past sages by placing oneself on their graves while performing certain meditations. In fact, Rabbi Yosef Chaim of Bagdad did this while on the grave of Benayahu ben Yehoyada and that's why so many of his Torah treatises bear some version of this saintly individual's name.
I hope this helps you out.