” Dr. Elman holds that most of the Minhagim of Niddah are taken from the Zeroastrians. (He may not be wrong on that entirely and it’s hinted in the Gemara, but he takes his opinion way, way too far.)”
And why would that be a problem? It is clear from anyone who has completed masechta niddah that the practices became much more stringent over time. And he has documented Persian practices in this area. Chazal had the authority to institute halachic stringencies for whatever reasons they wished, and some such examples are documented in their own writings. Beyond the pale would be to say that these changes were no longer binding today.
“everything found in the words of the Sages whether in halachos or agados of the Talmud or in the
Midrashim, are all the words of the Living God”
That is not the same as saying that everything in the words of the sages needs to be taken literally. Rambam, Ramban, and Rabbi Avraham ben HaRambam explicitly said that was not necessary.
“Chaza’l knew scientific information which wasn’t known to scientists until hundreds of years later”
Examples?