Okay, since you don't like my example, let's use yours. "if the deity came down from on high to tell you that you were going to make decisions that result in you wearing underpants on your head when you die..." If this unlikely scenario were to take place, according to the Muslim concept of free will, you WOULD be able to make decisions other than those that would lead to that end result. But God would already know beforehand his informing you would have that result. So even if you ended up not having "underpants on your head when you died" God would have known that that would happen beforehand.
For example, if God were to tell you that tomorrow someone will ask you such and such a question and you will answer "yes." Then when tomorrow came you would have the free will to answer "no." But if you answered "no," God would already know that that is what would happen.
The thing is, God does not do such things, especially not on an individual basis and never with enough preciseness for the cause and effect to be recognized beforehand. So there is not really that much confusion in practice. You see, by the actors in the events, that is us humans, knowing what is going to happen, that knowledge changes the system resulting in a different thing happening and if they find out about that, again that knowledge changes the system, etc... ad infinitum. An infinite loop. As long as we are kept in the dark, there is no problem because the predestination does not interfere in the free will.
Consider this example: A man was walking in the market when he sees someone noone else can see. He recognizes the person as the Angel of Death and recognizes a surprised look on his face. In shock, and fearing for his life, the man consults an advisor who tells him to leave for a distant country immediately. The man does so and the day he arrives in the distant country he dies while walking through the marketplace. When the Angel of Death comes to him, he asks him why he had appeared surprised when he saw him the first time. The angel of death replies that he was surprised because he knew that he was supposed to take that man's life in the far away marketplace so many days later and he didn't understand what he was doing so far away from there.
In this example, the man of his own free will chooses to travel to the distant country, but that he will do so is already known.
In short the Islamic concept of free will and predestination is that both exist but they do not tamper with each other. They are completely separate phenomenon running parallel to each other, arriving at the same location but without having any affect on the other. If knowledge of predestination were to interfere in the life of the one with free will, you would have the problems that you keep referring to.
I love latin!
Utinam barbari spatium proprium tuum invadant!
Sorry, couldn't help it...
In your interpretation of my example God lies. If the diety told the truth that the individual was going to die with underwear on his head, then that individual is going to die with underwear on his head and he has no choice in the matter. No choice = no free will. By simply existing, a deity with full knowledge of the future prevents anyone from having free will because within his (absolutely correct) view the story is told in only one way. The peons may feel that they have free will but they don't they cannot make choices outside of the way it happens.
Look at it this way: The events of yesterday have past, you know what has happened and it happens in only one way. This is the way a deity with the powers you describe would see all time. You know how your friend Michael is going to react to your comment about his shoes. Micheal in this history has no free will, he cannot in your rememberance do something different (he cannot in fact yesterday do something different), he has no free will yesterday. For a God of the sort you believe in, everyday is yesterday... the events are known actions by anyone and nothing can be changed -- there is no free will (and God realizes this)....