Assumptions V. Facts

A common tactic in fooling an audience is to present information in pieces and allow viewers to draw their own conclusions, which are wrong. Regardless of how reasonable an assumption may seem, film makers exploit this quite successfully so I was wondering if we could separate assumptions from facts in Prometheus? A few examples: How do we know the opening scene was earth? How do we know the Engineers are earth's creators? What did the Engineer drink at the beginning of the film? Since the black goo does not kill but only mutate living organisms, what is the purpose? How do we know the Engineers put the maps on the stone markings in the ancient cultures? How do we know the infected sperm from Holloway did not get in Shaw's blood stream and was restricted only to her uterus? I'm sure others can come up with much better examples so I hope those at least show what I mean. An in-film example was telling the audience Weyland was dead but there were many clues to the contrary, one of which was David talking to someone using the Stasis helmet. That was interesting partly because it showed direct communication during stasis which may have implications for Prometheus in Paradise.