No, that's not correct in English. Until can only be used to refer to a time point, nothing else. For example, 'I will follow you until we reach China' points to the time when the action is completed.
Yes, it is correct English. "I will follow you until China" means I will follow you until we get to China and then we will go our separate ways.
Can this also be translated as "I would follow you to China?" How hypothetical does this sound in Greek?
I would follow you to China=Θα σε ακολουθούσα μέχρι την Κίνα. The above sentence (the one of the excercise) does not sound hypothetical at all. It sounds as a statement.
Is μέχρι required in this sentence? Does it just emphasize the point, maybe like saying "I will follow you all the way to China"? It seems like Θα σε ακολουθήσω την Κινα means exactly the same thing.