Is there something impossible for God?
Nope. Not at all. However, just because someone prophesied something, does not mean it came from God....nor the Devil...just one man (a very holy man at that) saying something....sometimes a spoon is just a spoon.
Or do you think God always do things that look smart at our eyes?
No, not at all. However I think that sitting around and dreaming about Istanbul returning to Christian hands is a waste of time. If it happens, great. If not, oh well. What flag is raised over the city will not lead one person to Christ.
Stop fantasizing about trivial things and live your faith so people can see Christ in you. This whole topic is nothing but an exercise in vanity.