Did God need foreknowledge to know Hezekiah was going to die. no
Did God need foreknowledge to keep Hezekiah alive 15 more years . no
your Hezekiah verses are about whether or not the future is settled
God is prophesying the punishment and the unrepentance
and open theist say God is guessing that he may or may not punish and they may or may not repent.
Rev 16:8 And the fourth angel poured out his vial onto the sun. And it was given to him to burn men with fire.
Rev 16:9 And men were burned with great heat. And they blasphemed the name of God, He having authority over these plagues. And they did not repent in order to give Him glory.
open theism teaches God guesses
open theism accuses calvinism of having a god that is a stone idol
open theism has a god created in mans image
sorry I do not agree with open theism or calvinism
You might not agree, but you're arguing against yourself at this point, as I described. You need to consider what you really do and don't believe. I'm not hung up on the name . If you don't want to be associated with the term "open theism", that's fine. But what you described is a changeable future, and I don't know what else to call it.