Answer

A. I am not aware of such a source, aside from “do not deviate.” That is a source for obeying the sages, but not necessarily for the existence of the Oral Torah. But if they say that there is an Oral Torah, and our tradition says so as well, then apparently there is.
B. There is no identification here between knowing something and being that thing. Specifically regarding the Holy One, blessed be He, apparently one who is not Him cannot know Him, because of His greatness and infinity.
C. Any yeshiva where they study seriously. Go get an impression, ask questions, and decide. There are many such places. I don’t think it matters very much which yeshiva you go to. As someone once said: the question is what you will do in the yeshiva, not what the yeshiva will do for you. You don’t need to look for originality or a special style of learning. On the contrary, go for a place where they learn Talmud in the regular yeshiva analytical style, and do it critically.
D. It’s far too much of a mess—time, budgets, and endless headaches—and I don’t know how many people are even interested. From what I see around me, very few.
Best of luck.