1. you're right in saying that they were always tied, but they were nailed as well in some cases. for some reason, most modern depictions of jesus dont show the ropes, but they were undoubtedly there
2. most christians would tell you that the new testament would take precedence over the old, although i think this is a bit fishy as well
3. see no. 2

what you suggest is possible, but i dont think the romans knew or cared enough about judaism to try to introduce a messiah figure into the picture. chances are that mary and joseph were just smart people who wanted the notoriety of having the messiah as their kid and trained him how to act.