You are assuming that God doesn't have emotions. Are you saying that God didn't get angry with Israel, when the passage clearly says so? Are you forgetting that God made man in His image? Wouldn't it follow that emotions are part of that image? Didn't even Jesus get angry at the moneychangers in the Temple?
But this is not the only example in scripture where God changed His mind. Nineveh comes to mind.
Your reference in Maclachi 3:6 only states that God never changes, that is His nature never changes, but that doesn't imply that He cannot change His mind and execute justice or mercy. Many of the promises of God were conditional, as in Deut. 28, blessing or curse. And through Isaiah, God told Hezekiah to set his house in order for he was about to die, but Hezekiah pleaded with the Lord and gained another 15 years.
Ezekiel 36 is a prophesy that hasn't fully occurred yet. While Israel became a nation again, the full realization of the passage will occur when Christ comes again. But you must also realize that had they accepted Jesus as the Messiah the first time, they would not have been scattered. Jesus said concerning John the Baptist, "For all the prophets and the law prophesied until John. And if ye will receive it, this is Elias, which was for to come." Of course this is referring to Malachi where Elijah would prepare the way of the Lord on the Day of Judgment. Again, this is conditional upon Israel's reponse.
But what was Jesus' will for Jerusalem?
"O Jerusalem, Jerusalem, thou that killest the prophets, and stonest them which are sent unto thee, how often would I have gathered thy children together, even as a hen gathereth her chickens under her wings, and ye would not!" - Matthew 23:37
Oh my! Jesus' will was usurped by Israel's unwillingness. Fancy that.
James also says that we have not because we ask amiss, that we might expend it on our own lusts. But James also says to lay hands on the sick and pray for them and they will recover.
Concerning David, as long as he kept close to the Lord, he was blessed. But he later committed some pretty horrible sins and his kingdom suffered greatly from it. Was that God's will?
So if I pray for salvation, will that change come?