Engineer Society - 3 BILLION+ YEARS OLD?!

I realize this question has probably been asked before, but I'm a noob, so please don't kill me. [In addition - in case there's any question - SPOILER ALERT for the rest of this post]. I loved the film, but of all the unanswered questions and/or possible "plot holes," the only thing really giving me trouble is the question of how old the Engineers are. Assuming the sacrificial scene at the beginning is the Engineer society seeding life on Earth, that would put them - according to our fossil record - at visiting us 3 billion years ago... Of course our records could be wrong, I suppose, but I wouldn't have thought Scott was going in that direction. (1) Even under the best of circumstances, is it reasonable to assume societies can last that long? (2) If they do, shouldn't they have gone through some pretty amazing evolutionary stages themselves by then? Would they really look exactly the same? And their technology... while the ship at the beginning did look different than that which we see later, and the Engineer we see was not wearing the exo-skeleton armor we see later, it doesn't appear that they've changed as a species. (3) Would a plan - any plan - to engineer life or anything else - still be important to a society after 3 BILLION YEARS?! Sure, Xenomorphs are a great weapon, blah blah blah - but wouldn't 3 billion years be long enough to create far superior weapons? A smart-laser that can instantly end all organic life on a planet without damaging anything else? Or a beam that instantly renders all life on a planet infertile, perhaps (haha - think about it). Again, I loved the film, but this question bugs me far more than anything else.