Yosi,
1. His name was Hillel Rivlin. One of his descendants was Shlomo Zalman Rivlin, who published it without the more esoteric parts but did not write it. It was accepted as authentic by Rav Teuchtal, Rav Tzvi Yehuda (Kook) and Rav Menachem Kasher.
2. The descendants of the Gra are numerous. It could be that some drifted into NK but they are actually a branch of Satmar.
3. How many proofs do I need? Do I have to repeat each one 400 times like Rav Preida (Eruvin 54b).