Attempting to fix the plot holes

HI. I'm new here. I watched Prometheus twice and generally think it is a great film. Some of the (smaller) issues I had after the first viewing didn't seem as bad the second time around (such as the foolish characters and lack of character development) but the bigger issues and theories left to the viewer to work out, however you frame them, do not adequately fit the motives of the engineers. A friend of mine likes to believe that the sacrifice engineer at the beginning was on his home planet, not Earth and I when I thought of the film with this perspective it was easier to find motives for the Engineers - for a bit. (However, it was Earth. Ridley Scott has confirmed, and the majority agree (including me). So after playing around with the Engineers motives - of not trying to destroy Earth - how about this: The Engineers created life on Earth [i] in the beginning. [/i] Fast forward to 2000 years ago. The Engineers are seen escaping LV-223. What were they escaping from? A Xenomorph Alien (similar to the goblin one or the one on the mural inside the room with the giant head, makes sense. But how did they create it if it took a [i]human [/i] host to create the streamlined Alien beast? My theory is that they seeded life on earth to create 'guinea pig' life - a race similar to themselves genetically - to experiment mixing the 'goo' and other gestating creatures with humans. They did this over hundreds of thousands of years. Earth was an experiment/laboratory based on their own DNA. Every so often, they would return - hence the cave drawings around the world - for new specimens to test. They got closer to creating the perfect Xenomorph 2000 years ago and may have wanted it for the same reasons that the Wayland-Yutani corp wants it. However they could not contain it and it wiped them out. The last ships in the pyramid were set to go to Earth for further specimens 2000 years ago but never made it due to the Alien outbreak Or, The ship might have been programmed to go to Earth 2000 years as an escape route for themselves (as it sustained life) They may or may not have been going to destroy all of mankind. (they probably were) At least one ship got away but was carrying an alien or facehugger on board (the one that crash landed on LV-426 - which was dormant for 2 millennia until the Nostromo came across it. The classic 'Alien' we all know and love had probably already been one of many creations by the Engineers (some goo + some engineers + some human). So when the Engineer meets the Peter Wayland and co, he thinks 'these things (humans) are just lab-rats. WTF are they doing here? kill, kill! this is the last thing I need. Do this help explain any other parts of the plot?