The following is a response to an email I received about Molinism. Hopefully it helps!
1. Did God create S in such a way that He would definitely commit A? If so I don't see how that's any different that Calvinism.
You're right that it would be some kind of Calvinistic determinism, assuming that by your sentence you mean that God's creation is a factor in the counterfactual truth of "S would do A." So the answer is "no"; on Molinism, whether or not S would freely do A is independent of God's willing some set of circumstances in which S does (or does not) A.
2. If God did not create S in such a way that he would definitely commit A but only actualized the world once He knew that S would commit A, there appears to be a logical moment when God did not know what S would do (whether or not he actualized that world seems irrelevant).
That's right. There is a logical (not chronological, keep in mind) moment in which God does not know the counterfactuals of creaturely freedom (namely, the moment of necessary knowledge). "Logical moment" is just a technical term we use to signify logical relationships; it signifies what God has to know first in terms of priority, or what needs to be known in order to know something else.
3. If we get rid of the "once" in #2 above and instead say "actualized the world He knew that S would commit A", I don't see any difference between that and Calvinism either - I don't see where S had any choice.
Well we would have to understand that in the second moment, God knows the counterfactuals of creaturely freedom, which state "In C, S would freely A," where C is a fully-specified set of circumstances, including the history of the entire world up to that point. If God is actualizing something by exercising his will (which he does prior to the third moment), he is just stating " 'In C, S would freely A' is true and actual," as opposed to " 'In C, S would freely A' is true." So long as there's no problem with the counterfactual being true by itself, I wouldn't think there's any particular problem with God deciding that this world, as opposed to another, is the actual world. Now, if one says that he objects to the state of affairs of counterfactuals being true, then it becomes clear that the "God part" of the objection is irrelevant, as the problem is not with God and his activity in the world, but with the truth of counterfactuals themselves. But that is a different story! :)
Hope that helped!