Predestination and prayer

A stock objection to Calvinism is that it renders petitionary prayer pointless. My stock response is that in Calvinism, prayer and answered prayer are means by which the future is realized. To put it counterfactually, some events will happen in answer to prayer that wouldn't happen absent prayer. That's entirely consistent with predestination. However, I'd like to go beyond that to address the objection from a different angle. 

i) The Bible contains a number of blanket prayer promises. There is, however, a class of unanswerable prayers, and that's contradictory prayers. That's true for Calvinism and freewill theism alike. So there's an implicit exception to the prayer promises in reference to contradictory prayers. That doesn't call into question God's sincerity since God can't perform contradictions, so it should be understood that contradictory prayers aren't included in the promises. 

ii) Mind you, the Calvinist God might prearrange history to reduce the number of contradictory prayer promises. However, that has to be counterbalanced by other considerations. A world history with more unanswerable prayers might be better overall than a world history with fewer unanswerable prayers. Prioritizing circumstances to minimize contradictory prayers would marginalize other goods. 

iii) That said, freewill theism adds another class of unanswerable prayers. According to freewill theism, God has made prayer promises that he cannot keep because God's ability to make good on his prayer promises is often conditional on the independent cooperation of human agents. This means that in many cases, God can't grant your petition even if it's a perfectly legitimate request. In Calvinism, God is able but unwilling to answer some prayers while in freewill theism, God is willing but unable to answer some prayers. 

In that regard, freewill theism calls into question the sincerity of God's prayer promises. Unlike contradictory prayers, which are necessarily unanswerable, these prayers are contingently unanswerable. As a rule, God won't answer them because he must "violate" the freewill of human agents to grant the request. 

If freewill theism is true, there must be a vast number of prayers that God cannot answer if granting the request hinges on the independent cooperation of all parties to the petitioned outcome. Consider prayers for salvation, prayers for persecuted Christians, prayers to halt the wicked from harming the innocent. Since all concerned parties have libertarian freedom, God can't grant the request if they refuse to cooperate unless he overrides their freewill. 

So that creates an enormous loophole in prayer promises. It's unclear how the prayer promises were made in good faith when so many situations we pray about aren't actually covered by the promises. The God of freewill theism was never in a position to deliver on prayers that fall under that incalculably huge, but unstated class of prayer requests. 

It's like offering everyone who shows up at a certain time and place a free laptop, when only 30% of those who show up get the laptop. If the God of freewill theism knows that a vast number of prayer requests are automatically disqualified, should he not word the promise to make it more realistic?  

N.B. Don't compare that to the relationship between special redemption ("limited atonement") and the well-meant offer of the Gospel. To continue the illustration, 100% of those who show up for the laptop receive a laptop. Everybody responding to the offer gets what was offered.