In the spirit of open-mindedness, I will now imagine that it is my job to make a case that the 2 Kings 20 text can be reconciled with the "traditional" (non-open-theism) position. I do this for the sake of stimulating further substantial debate - I do not agree with the conclusion of the following argument , but I hope it can serve as a catalyst for you opponents of open theism to build a stronger case.
Here goes:
It cannot be denied that God tells Hezekiah (through Isaiah) that he will not recover. It cannot be denied that Hez indeed does recover. If God knows the future, this makes him seem to tell Isaiah a falsehood about Hez's future - something we really don't want to attribute to God.
Perhaps a solution lies in the limitations of the audience for whom this account is intended - you and me. Perhaps God has to portray himself as a human with limited knowledge of the future in order to deliver his main theme. So the important point here is that God portrays himself as having the limitation of not knowing the future in order to help us understand his message, even though he does indeed know the future as fully settled. Being humans, we need to have the story told to us in a way we make sense of, and perhaps God needs to present himself to us as a limited human in order for us to understand the main point (whatever that happens to be).
It may not be fair for people like Drew to suggest that this text forces us "traditionalists" to have to choose between a God who knows the future fully and one who misrepresents the truth when he says "you will not recover". Perhaps God really does know the future as fully settled and his "you will not recover" statement is not a falsehood because it was a necessary literary device used in service of transmitting the main theme to the reader, a reader who cannot easily comprehend an omniscient God but can perhaps understand things better if God represents himself as limited.