Here are some random thoughts I have on prophecy, not sure if it fits in:
Many prophecies, such as those of Jeremiah, Isaiah, and Jonah, are were open, they were conditional, they were contingent.
What is the purpose of a prophecy? Is the purpose to rub someones inevitable future in their face? Or is it to alter and change the course of the future, to avoid one future possibility and secure another future possibility?
I believe that the bible, and all the prophets believed, the latter and not the former. That is why Jonah didn't want to go to Nineveh, because he believed the future was open, and that is why Jeremiah lamented over Israel, because their captivity was avoidable and contingent.
Prophecies, serving as a warning, all presuppose the openness of the future when it comes to some things. Prophecies that were given, to warn people or groups, in the hope of avoiding the foretold possible future, all necessarily presuppose the future being open to possibilities, to alternative or multiple outcomes.
And of coarse King Hezekieh believed that prophecies could be contingent, and God's mind and plan could be changed, or else He wouldn't have sought the Lord in prayer to change His plans. King Hezekiah was not a believer in a fatalistic certain system, but obviously believed in an open system.
Other prophecies are certainly not contingent, but are closed, because they are derived from the determined counsel of God, not the contingent choices of men, such as the death of Christ, which was determined by the councel of God.