Ok. I've been following this for a while, and I think I understand what you are getting at. I also think I might be able to answer you question in a better way. Correct me if I am misunderstanding anything.
Let's begin with Tom. Tom's mom has cancer. Tom prays that her cancer is cured, if God wills it (His will be done). You seem to be saying that this prayer is a waste because either A) if God's will be done, then the same thing would happen even if Tom was silent, or B) by answering prayers such as this, God changes the future and thus surrenders His omniscience.
I do understand how asking someone to do something that they are already going to do can seem like a waste. I think you are putting too much emphasis on the percieved end result of prayer, rather than its process and secondary effects. If I ask you for something you were going to give me anyways, my asking is a waste. But if my same asking makes you feel more appreciated, or improves our relationship, is it still such a waste, even if the same result (me gettting something) is achieved? This is my response for A)
I do have one more problem with A) though. There isn't any way to know whether the result would be the same at the end or not. We don't exactly have a way of going back and testing that theory. And if God's will was really that simple, would anyone have cancer? Would anything bad happen, for that matter? So it seems to me that His will is more complex than that.
B) appears to rely on the existence of the multiverse. By that I mean, that for every possibly choice, there is a possible world where that choice was made. If, however, God knew before creating the universe what every possible choice was, and also what the end result of said choices were, and then only actualized one of these many worlds, this argument falls apart. It means that God already knew that Tom was going to pray, what he was going to pray for, His own reaction, and every event which would continue to happen afterwards til the end of time. He maintains His omniscience.
Please inform me if I am incorrect on my understanding of anything. I will happily work with you until we can come to a satisfactory answer.