I think Zoroastrian dualism is more usually understood as cosmological rather than metaphysical. That is, Ahura Mazda is indeed omnipotent and perfect in Zoroastrian thought. He is the only God. Angra Mainyu is not another God. He is an evil spirit, like satan, in no sense equal to Ahura Mazda. Zoroastrianism simply emphasises the cosmological role of the duality between good and evil even beyond what is seen in the semitic monotheisms and other faiths; it is, though, a duality that is overcome and is not fundamental. This, at least, is my understanding of the general Zoroastrian view (contrary to some claims, one with deep roots in the faith). There certainly have been radical or metaphysical dualists amongst Zoroastrians, including the important theologican Mardan-Farrukh, but it has never been the dominant position, as it may have been for Manichaeism, some radical offshoots of Zoroastrianism, and certain strains of what is called today Gnosticism.

No, he is much as the Christian God is, at least in the dominant traditional interpretation. In the Yasna ceremony, the principal Zoroastrian rite and its accompanying liturgical material, his names are recited, and these include Harvesp-tawan and Harvesp-Tavaan, or almighty and all-powerful. It is the bounteous spirit, of which in Yasna 47.1-6 (the Spentamainyush Gatha) Ahura Mazda is called the father, who is seen as in some sense the equal to the evil spirit. The problem is that not only is there a diversity of opinion in traditional Zoroastrianism, but that a lot of Western study ignores the tradition of Zoroastrianism and tries to force it into various procrustean beds depending upon the different scholars' perspectives. As the Zoroastrian texts can be somewhat obscure, this creates even greater problems than it does in historical research into Judaism and Christianity.

It is much like satan, or, indeed, human evil. God permits it. Angra Mainyu is, in a sense, a personification of all the downward tendencies, the tendencies of separation from God, in the cosmos.  

I'm unaware of pre-Muslim Zoroastrian theologians in anything like the Scholastic mode. There were certainly holy men and authorities. Iranian Islam and mysticism, like that of Suhrawardi and Mulla Sadra, has a Zoroastrian influence.