I think you're using this passage for a different purpose than it was intended. Isaiah was reassuring Israel about his commitment to his covenant people. The whole verse is
declaring the end from the beginning
and from ancient times things not yet done,
saying, “My purpose shall stand,
and I will fulfill my intention,”
Pretty clearly the reference to ancient times is the beginning of the covenant, and God is saying that he will fulfill it. This is not a philosophical statement about the knowability of the future, but rather of the constancy of God's purpose.
Surely it's clear what I meant. I'm not sure that open theism claims God has enumerated every possible chain of events, but rather that he can always find a way to accomplish his purposes.
I already noted that open theism seems to assume that God is within time, and from a scientific point of view, it seems most likely that he's outside of time. Basically the problem is that time is an attribute of the universe. (It's one dimension of a multidimensional space that doesn't appear to exist "before" the big bang.) If God created the universe, he must exist outside it (though of course he is also within it). It's hard for me to see how someone outside the universe could be temporal.
It's just that Scripture describes God as acting as an agent within time.
The problem is that we don't have any idea what it is like to exist outside the universe. Although our time seems to be part of the universe, perhaps there is still some analog of time outside it. Or perhaps Calvinism is true, and the whole way the Bible speaks of God interacting with us is accommodation.