I’m finding it challenging trying to comprehend the Orthodox stance apropos Saint Augustine.
That's true, and it is evident. You are making a lot of confusion on the subject. I would suggest you to have a talk with your spiritual father, as you are a catechumen and he'll obviously be glad to explain you everything before you're baptised.
Anyway, let's have a look to your conclusions, and see which are right and which are wrong.
1) Adam and Eve were, so to say, 'real'. Most Orthodox, as I and many others here, confess two historical persons, yet many believe in Adam and Eve to be fictional characters created to show the passage from an initial Golden Age of innocence to a Fallen Age of sinfullness in the primordial history of humanity. How this happened, this is subject to private interpretation... my mind finds it easier to keep the two characters as real as possible to have better in mind the possible allegorical meanings too, but somebody may think it different. To share questions and doubts on this, there's a billion threads already open on the subject, in particular a recent one on Adam and Eve.
2) You say that, in your eyes, the introduction of sin was inevitable, and so that God is somehow an efficient cause of evil. That's wrong. God was the direct cause of FREE WILL. The purpose of free will is to make humans freely love God, and not to hate Him and introduce sin. Still, free will implies also the freedom to sin, yes. If outside of a shop you see the words "Free Entrance", are you obliged to enter? Of course not. Free will admits the free choice to accept God's love or to reject it. For our spiritual wealth, it is natural accept it, because it costs a bit must gains a lot, when the prize is life and joy eternal. Nevertheless, there exists the free choice to welcome evil, i.e. to reject God's gift. Adam and Eve were born with the gift of immortality, and in the Garden of Eden they were proving it temporarily. The only condition was to persevere in God's love, a thing which didn't occur. While not concupiscent by themselves, they were tempted from outside. The snake is a different character then Adam and Eve. He is the agent of our spiritual and physical death: Jesus himself calls him "Murderer since the beginning". Satan had been given himself that choice, and he decided to sin, dropping down from heaven all his legions of fallen angels with him. On earth, he did the same with us (or better, our ancestors). Like angels, we became sinners by nature - i.e. concupiscent. The contagion passed through all generations unto us, and we are delivered (partially) from it baptism.
3) -Did Adam have a choice not to sin? Yes, he did.
-Why did he sin then? Because he wanted to, as we also do in our lives. Only that Adam chose something which was contrary to his own original state of grace, while his descendants act according to their fallen nature.
-Can we choose to work against our fallen nature? Yes, with God's help, we can. Adam did the opposite, so we can reverse the process. God provided a way in Christ Jesus. Sin is not inevitable... it is just extremely difficult to eradicate. May the Immaculate Theotokos be our example!It is right to say we need a Saviour, but it is wrong to say that we can't ourselves. We can save ourselves ACCEPTING Christ our Saviour. Jesus doesn't choose whom to save (a Calvinist opinion...). Jesus offers His grace to ALL, but even if we are damaged by the effects of ancestral sin, we can still find out that God is calling us, and accept his help. This way, God will offer his hand to us, and he will lead us on the way to salvation.
Hope this helps,
in Christ, Alex