Kenneth R.
Cináed
The only thing that is in conflinct between Alien and Prometheus is the name of the planet, since I'd say it's pretty safe to assume both movies are set on the same rock. I could think of reasons why they would do this on purpose, but they'd be a bit contrived.
I can't recall what exactly would be lost if ten seconds were cut, but personally I would have ended it without revealing whether Shaw and David got the other ship working. Showing them heading towards it and ending it there would have been perfect.
I don't think it's the same planet. It could be in the same system though, or nearby. This may not be the Engineers' only installation. Who says the humans got the right directions when they aimed for that moon?
You're bolder than I even. Yes, that would work too. Anything that leaves it ambiguous. The bit at the end felt like the spoon-feeding hollywood gag that the film so expertly avoided up till that point.