I've heard it said that the valleys and mountains being made plain refers to the mountainous land between Babylon and Jerusalem; that the way back from exile would be made easy. I'm not sure what it's all about, but I suspect it will be one of those bits of prophesy which has fulfillment in different ways in different eras, but ultimately in Jesus.
I agree with this - the prophecies often have multiple fulfilments - talking both about the restoration of Israel, the coming of Jesus, and the future coming Kingdom of God.
You may be right about the Romans - the Pax Romana (I think that's what it was called!) allowed free travel throughout the empire and did indeed mean the gospel probably spread more than it would have done without the empire. I was thinking about this recently actually, Christopher Hitchens (an atheist) often makes the point in his debates that God waited for thousands of years before sending Jesus and saving humanity. It seems to me that one answer to that is, if Jesus had come before the message might not have spread to all humanity.
Having said all that, I don't think you can read too much into prophecy, I suppose the important thing is the message rather than the specifics!