I'm led to believe the wheelchair GUY -(thanks LarsVader lol)- is born of Noomi Rapace after being infected somehow. It could be that the engineers, like the aliens, are also parasitical beings that need a host to come to life and complete their doing, and have also accelerated growing proccess like the aliens, though the engineers don't kill their hosts when they are born.
I'm starting to think that the aliens were first some sort of serpent beings, very similar in appearance to what the silver banana head of the first alien is, but of course bigger in size and no biomechanical body like the Giger-alien; then when one of the engineers, maybe the bad one, mixed Its own DNA with the alien species DNA, the once serpent-like beings the aliens were gained a biomechanical body structure, common to the engineers/jockeys. The rest would be up to the host, with the chestbuster stage being the step when the DNA of both jockey and alien creature is also mixed with the host DNA, with the outcome we have seen in the movies being an adult alien. I think chestbusters are also a farcry of the original form of the aliens.
Scott also said that each of the eggs aboard Derelict contained Jockey's DNA, so maybe this fusion between the two species was carried out entirely in the chair while the engineer was plugged to It, being those eggs produced by the very chair while the engineer is still plugged to It, laying them one by one in the chamber.