It is precisely because religions INCLUDING Zorostrianism have become about control that for me you have to find your own truth, you cannot rely on any present body to know anything of truth because that isn't their purpose. What is more, the further back you go, the more likely it is that these things have been molded to a particular groups needs again as a means to control directly as part of the religion - it is basically like old school patriotism...
Any identification with anything will block your route to truth, all identification is utterly false, and while I respect Zarathustra it is HIS experience which he talks about, it can never be yours or mine. If we try to emulate another persons experience we are making our existence null and void, just a carbon copy, and that is absolutely wrong. That doesn't mean we can't take some things from his words along the way, but ultimately religion must transcend texts all together, it must become your daily experience, and in that there is no thinking about other religions at all, there is only living life.
It is almost certain that Hinduism, Zarathustrianism and Mithraism have an ancient ancestor in common for the simply reason that each talk about the same certain Gods. The very fact they have split shows there is something basically wrong with their ideas about oneness. It is why I love the Sufi's, because they are trying to bring everyone back together, but I love Zen more because they are trying to bring about uniqueness in everyone, free from all concepts where Sufism still works with things like God and love - both are merely ideas, you don't know their truth.
Yet, you say religion isn't about finding truth, it seems then that you are fine with their lies... nothing wrong with that but that isn't my purpose in delving into them. I am not interested in a society of slaves, my whole interest is that everyone find absolute freedom.