2. There is no evidence that anyone at all celebrated Lag B'Omer before the
17th century. (Please correct me if you have evidence
otherwise.) No less an authority than Chasam Sofer was strongly opposed to Lag B'Omer celebrations. He argued that one should not make a new Yom Tov that is not based on a miraculous event, that has no basis in Shas and Poskim, and that is based on the death of someone. (See here for links.)
3. When Lag B'Omer falls on motzai Shabbos, this causes an immense amount of chillul Shabbos, that would not otherwise have occurred. See this account by one charedi firefighter who was forced to leave his family on Shabbos in order to go to work. As discussed last week, due to this problem, the dati-leumi rabbonim ruled that one should not make a bonfire on motzai Shabbos. This ruling is not supported by charedi rabbonim.
4. It is widely accepted in the Orthodox world that the Zohar, which Moses de Leon published in the 13th century, was written by Rabbi Shimon bar Yochai. It is further widely held that to question the authenticity of the Zohar is heresy. You Don't Mess With The Zohar.
5. Nevertheless, Chasam Sofer was of the view that the vast majority of the Zohar was not written by Rabbi Shimon bar Yochai, but was instead written much later. The standard view amongst frum people with academic training and/or non-dogmatic attitudes who are knowledgeable about this topic is that the Zohar was largely written by Moses de Leon, albeit incorporating older traditions to a lesser or greater extent. See this lengthy article by an anonymous charedi rabbi discussing many problems with the Zohar. Rav Ovadiah Yosef said that because of the serious questions that arise with attributing the Zohar to Rabbi Shimon bar Yochai, one cannot call someone a heretic for rejecting his authorship of it. See too this article by Dr. Marc Shapiro mentioning other authorities that rejected the antiquity of the Zohar.
(My explanation about the deletion of the posts about the Kaplan Affair will be posted at a later stage. There's nothing nefarious about it.)