Perhaps it appears as though this matter lies in the same "mystery" department as "Where does the universe end?", or "What were the Nephilim?", etc., but I believe this is one of those matters which is in fact disclosed in the Book.
It, of course, returns to the ever-troubling matter of whether there exists such a thing as "free-will" in any being except God Himself. If one is persuaded that the Bible teaches that there is such a thing, then it would follow that this matter is a mystery and that certainly the Noahic episode is evidence of this.
However, if one is persuaded that the Bible does not teach that men are "free", but rather God manages all things, then it would follow that God, alone, is the scripter of all events of the universe, all actions of history, and all matters which transpire. As you might suspect, I find this latter perspective all over the Scriptures.
What to make of God's calls for man to "decide" something? This is of course simply the Author writing into the script a situtaion wherein a character is told to decide. The outcome, however, is always determined, always planned, always the exact expectation (even the work) of the Author. This is how the Father arranged for the Son to be executed according to His, "foreknowledge and pre-determined plan". He knew it was a necessary part of the drama to save us from ourselves (and hell, too, of course).
Hundreds of seeming "free events" are pictured in the Scriptures, only to be later revealed as completely controlled by God. Even God's call to Adam, "Where are you?" was not actually a request for Adam to fill God in on something He did not know, but rather just a part of the script displaying the renegade nature of man running off from His Maker. All things are occurring exactly as determined by our Lord. Perhaps this seems Hyper-Calvinistic; so be it. The Book tells us this is the case.
You are most correct that God guides and directs the affairs of man. My statement I suppose lies in how He brings that about. I suspect that sometimes He uses demonic forces to move us into accomplishing His will. Other times He understands our nature so well that He can effect His plan just by knowing what we'll do. Probably the most famous case of these two examples is Adam and Eve. Certainly God put the tree and the serpent in the garden and then issued the command. The serpent deceived Eve into sinning and falling. Yet, there was no serpent to tempt Adam. Only the lust of his own heart which gave way to sin. God knew which direction he would go and He created the circumstance. But more important He had to show Adam what he was actually like. It wouldn't have done any good if Adam was deceived for then he would never know that he would trade Paradise over a piece of fruit.
God used two different methods to accomplish this outcome. It was clearly within God's will that both would fall-yet they fell in different ways. But, as scripture teaches us, Adam's fall was the greatest of all. Had he not been presented with this choice, we would never have know our rebellious nature or the love of God to want to reconcile His elect to Himself.