Speaking from the non-Calvinist viewpoint:
God knows what we are going to do (everyone agrees on that). He knew Mary would say yes and be willing. But he didn't force or manipulate such an outcome. It was her own free choice.
I think the first thing we have to do is realize we are dealing with philosophy. We are trying to know about God "through the back door" rather than through Christ - so it is more philosophy than theology (as is Calvinism).
If God knew and made his plans on what he knew then we have a problem of process. God is now looking, planning, and deciding (which is clairvoyance....peering into the future....not omniscience).
All sides have similar things that can be picked apart because we are dealing with human wisdom anout God rather than what God has actually revealed.
I see this as a way to understand one another - not to persuade the other to our philosophical view.
If you want divine truth then we have Christ. Don't look to Calvinism but to Scripture. We can have divine truth (the "deep" things of scripture) regardless of our theology.