Hello, my friend!
I was once a staunch Calvinist, and I know exactly where you're coming from. It's difficult explaining Orthodox theology to someone of the Reformed tradition, as there are so many differing paradigms.
For example, to the Calvinist, God's reason for creating us is for us to worship Him. But that means that God has a felt need for created beings to tell Him how great He is on a continuing basis. Another one is that created beings can have an eternal effect on Him; that WE can make God infinitely angry; that the problem with sin is that it affects GOD!
So I feel your pain.
Depending on how you look at it, Jesus didn't have to die (and the Calvinist will affirm that as well). He could have left us in our sin. But on the flip side, for an Orthodox christian to say "God could have left us in our sins" shows an ignorance of God's character. God is Love. He is Eternal Communion. When God created a universe that was "other" than Himself, He created it to be in union with Him. When He created man, He created man to be in union with Him. He created a race that can freely reciprocate His love.
God is Who He is. He is Love. He is Communion. It is His character to pour Himself out and fill things with Life. When we sinned, we didn't affect God. We didn't cause a change in Him. Rather, we caused a change in ourselves. We caused ourselves to move away from God, and to distance ourselves from the Source of Life.
Everything that Christ did in relation to us and to creation was salvific. His Incarnation saves us. His baptism saves us. His virgin birth saves us. His suffering saves us. His death saves us. His resurrection saves us. His ascension saves us. Him pouring out to us the Holy Spirit saves us. Christ did what was necessary to restore us to total communion with God. His death was part of that picture.
Our sin had many consequences for our race. One of them is death. And death has many facets. One of those facets is that death breeds more death. Another is that we are afraid of dying; we are afraid of not living forever. So we try as hard as we can to preserve what we have. We over eat for fear of famine. We are over-sexed for fear of our genes dying off. We become angry and kill for fear of someone getting in the way of our legacy being furthered the way that we see fit. And ultimately, death would result in us being eternally alienated from God, the source of life. So Christ entered death, so that by joining us in death and rising from the dead, we could join Him in His Resurrection. By Him dying, death, the chief enemy that resulted from sin, might be destroyed and we could have eternal union with God again.