Milhouse –
Due to the constraints of time, shorthand responses point by point:
1. I understand it may be surprising, even shocking. I was also startled when I first saw it. I know this is not what we learned in Yeshiva. Nevertheless, it is abundantly clear to the objective researcher that many Rishonim and Acharonim held this way. Are you labelling Rav Shamshon Rafael Hirsch an apikorus?
2. The Rambam labels both of the first two groups fools, but does not label either one of them as heretics. He does label as heretics those who deny Torah Min Hashomayim, including both Oral and Written Torah, but seems to define Oral Torah as the “explanation of the Written Torah, such as how to [perform the Mitzvos],” which is precisely the definition given by the Ramban. I do not defend heretics. I am merely presenting the view (which is widely held among the greatest authorities) that Midrash (and, according to many, Agadeta) are not part of the Torah which was given to Moshe at Har Sinai.
3. The passage in Sefer HaVikuach is quoted by some of the greatest Acharonim, including, for example, Chasam Sofer in Teshuvos, none of whom (to my knowledge) advanced your argument. It is true that, there are some recent Rabanim who take your approach and discount any points made in a Vikuach. To me, this is an outrage. Do you really mean to allege that the Ramban promulgated apikorsus in defense of our Torah?! (Quite aside from the fact that his opponent in that debate was an apostate Jew who was rather learned and would have been able to destroy that particular argument if it was not the real truth). This is very different from the Gemaros you reference, for obvious reasons.
4. It’s rather clear that the Ibn Ezra, Ramban, Radak, and many others have no problem disputing the historical facts asserted in Midrashim. If you want to say, “Well, Vashti didn’t literally have a tail, but the Gemara is teaching us a lesson in Middos,” that’s absolutely fine. It is, to paraphrase the Ramban, just like when a Darshan embellishes a story in Chumash to teach a lesson. We are free to accept it or not to, each according to his understanding. Of course, this is not to say that the lesson is not valuable; just that the Derashos of Rav Tanchuma in his day were not (according to this opinion) any more Torah Min HaShomayim than those of Rav Shalom Schwadron in his day. I am not saying it isn’t true in the sense that it is instructive, or relevant in the sense that it addresses us, just that this opinion holds that it isn’t an “inherent part of the Torah.” I am not stupid enough to pretend for a moment that I approach the greatness of intellect of Chazal. Of course, when they speak, we should listen. It is the foolishness of the Rambam’s second group to think that we are on anything approaching equal footing with Chazal. That is not the question at hand. Our discussion is about whether Midrash is part and parcel of Torah Min HaShomayim, not whether we should learn the lessons it teaches.
I notice that you did not address Rav Shmuel HaNagid or the RItva.
For the record, if I recall correctly, the Chasam Sofer accepts the Ramban implicitly, and only enters into discussion about whether the Ramban meant to include Agadeta Gemaros with Midrashim, or if he held those were part of the Oral Torah.
There are those who attempted to sweep the letter from Rav Hirsch under the rug by disputing its veracity, but they ultimately had to acknowledge that there is no question that the letter was accurate. When pressed about it, they reputedly responded, “Rav Hirsch iz nisht fuhn unzer Beis Medrash.” This to me is rather telling about their own intellectual dishonesty, but not at all about the actual discussion.