there is no reason to believe that the synoptic writers were "eyewitnesses," and plenty of evidence to conclude that they were not. The "earlier manuscript" you mention sounds a lot like the Q argument (which I happen to buy into). Q is partially responsible for Matt. and Lk. So is Mark. Q is likely an oral source, though, (as is earliest Mark). So we're talking about "stories that were told about Jesus." In that culture, that doesn't present a problem for historical accounts, because most historical accounts in those days were oral accounts (at least at first).