There was a thread on here about the Zoroastrian connection to the construction of the Bible as we know it. I think you are very correct with your view of the evolution of the concept of a God being " Good" and the Devil being " Evil".
The obviously early sources used in the O.T by the compilers have no concept of Satan as 100% Evil, it is only the stuff put in, or written after the Return from Babylon that contains it. "Job" of course is early, but the figure questioning god in this fable is obviously not thought of as actually existing as a "being". It is a long Metaphor, for teaching purposes, not a Book about the Spirit world to be taken literally.
There are Scholarly Papers available on-line that deal with the influence of Zoroastrianism on Jewish Theology, the Thread I mentioned I think has links to one or maybe two. Was the thread by Poster " How The Bible Was Created" ?? do a search, good luck ! The influence is undeniable, and of course is reflected in the Theology developed by Christian writers in the N.T, and later.