A few points from a former Calvinist:
1) This whole argument assumes that speaking of God's 'foreknowledge' isn't an anthropomorphism. I.e. it assumes that you can speak univocally about the Divine Essence. Put another way, for God to be able to know things 'before they happen', He would have to be subject to time, i.e. there would have to be a past, present, and future for God. This is where Fatalism comes into the picture. St. Augustine brought 'necessity' into his philosophical view of God's nature based on his pre-Christian Platonism. We however must reject that philosophical notion in favor of confessing the Truth that God is outside of all categories of time, that He created time, and is not subject to it. What does this concretely mean? Dunno. Can't conceive of it, because we're completely submerged in time (the created order). Welcome to Apophatic Theology.
2) Most of the Calvinist argument is predicated on the idea that God's Sovereignty over the created order is predicated upon His actually controlling/causing everything which transpires in the created order. But this doesn't follow at all logically, as they claim. For God to be Sovereign over the created order, everything in the created order must be subject to Him, meaning, He must have the Power to control every part of the created order. Having that Power makes Him sovereign, He doesn't have to exercise it in every case. Put another way, God is actually far more sovereign than Calvinists think. He's so Sovereign that He can give humanity free will, and still remain Sovereign. (As an aside here, imagine if we apply the Calvinist definition of sovereignty (complete control) to civil government, or to a husband's sovereignty in his home...we'd call that petty tyranny, so how does it not make God a petty tyrant?)
3) St. John Cassian is my patron for a reason.
Recommended Orthodox reading on this topic (in criticism of St. Augustine at the time he lived at this point) is Conference XIII. You can read it online here:http://www.ccel.org/fathers/NPNF2-11/jcassian/conferen/book2/conf13.html
Hope that might be helpful.