Who wrote the anonymous rationalist debunking of kabbalah entitled Tohar HaYichud?
I do not know who wrote Tohar HaYichud as it is written anonymously. However it is posted on the Mesora.org website which was founded by Rabbi Moshe Ben-Chaim and the board members are:
Rabbi Reuven Mann: Rabbi-Young Israel of Phoenix; Founder-Masoret Institute for Advanced Jewish Studies for Women, New York
Rabbi Pinchas Rosenthal: Exec. Dir. -Manhattan Jewish Experience, New York Former Principal/Teacher-Torah Academy of Suffolk County, New York
Jessie Fischbein: Jewish Studies teacher/Author, “Infertility in the Bible”
Rivka Olenick: Editor
Jonathan Herman: Contributor
There is a question from a reader regarding Tohar HaYichud which was responded to by Rabbi Saul Zucker.
Based on a post on this site, which lists these individuals as authors/contributors (but does not specify who wrote the aforementioned post, the content of Tohar HaYichud is a [summary] of Rabbi Yichya Kapach‘s case against the Zohar’s authenticity. I understand this does not mean Rabbi Kapach is the author per se.