If God can't change His mind, then He is powerless to stop His own will.
God changed His predestination/decree multiple times. One of the best examples is King Hezekiah, who God said would die but chose to extend his life.
Also consider when God sent an angel to punish David.
Although this is true:
So will My word be which goes forth from My mouth; It will not return to Me empty, Without accomplishing what I desire, And without succeeding in the matter for which I sent it.
Notice Without accomplishing what I desire. Also notice without succeeding in the matter for which I sent it.
If God's desire changes, His word changes. This is basically Jeremiah 18:7-10, 2 Chronicles 7:14.
The only ones exempt from this grace are:
-those who sincerely betray Him (not like Judas. If he had repented, he could have been saved)
-those who kill too much (i.e. Hitler, Osama). God doesn't want these saved but will have to (since they would inevitably repent) if one should betray Him.
-those who know repentance and don't do it until they die and expect to go to heaven.
-those who commit sins of knowledge willingly. It's in Hebrews 10:26-27, 31, Matt. 12:32, and Matt. 10:28.
-the Antichrist and his followers
That said, I think Open Theism misinterprets truth to mean that God is not Omniscient. God is Omniscient.