2 Questions/Thoughts

Well on your second thought, I kind of like the idea that there could have been lots of different specis. although, I don't think its the case, its just intersting

I think the Engineers might've used Humans in the stone age to experiment. Still yet to see a Xenomorph without the assist of a human in its birth. Except that Bull in Alien 3

The Sacrificial engineer was not on earth but on another planet. The engineers goal was to start a xenomorph race on that planet so it just shows how the engineers do their own things. And to your other question that planet was only a military facility that had urns of the black goo. Pretty much they're all the same. Only the engineer crew was there to destroy humanity but of course failed due to a virus outbreak.

This is where the movie is so ambiguous that we really cant be sure what the Agenda is or was, so anything goes until the franchise shows us otherwise. Yes there is no guarantee that the opening Scene was Earth, Ridley even said it does not have to be Earth. And that the scene simply shows how the Engineers seed their Genetic DNA to create life, what is interesting is Ridley said this is not the only way they can achieve this but its how they choose to do it and thus it has some religious and ritual element to the Sacrifice. I do however not buy that they would go for all that effort to create mankind to simply be used as Hosts for Xenos.... And Xeno is not the perfect lifeforms, it only perfect for its purpose to survive and kill, to which case we could say that Ants and Cockroaches are perfect life forms. Mankind can do so much more and has more potential. I would go the side of the Prometheus Myth or a connection and also how Ridley said that these Engineers had visited us and taught and upgraded us over the centuries. Ridley hints that it was only after they found mankind was not behaving in a way the Engineers wanted and they sent a Emissary down that we crucified that they then decided to wipe us out and create something new. I therefor think they created us for some reason, maybe do procreate there kind as they lost the ability to mate? or that they wanted to create us as their children or maybe so we can worship them and perform slave tasks, so pretty much like the Anunaki Story. At some point a selection of Engineers had found some other being to worship and tried to recreate said organism, and having created the Goo as failed attempt maybe? They decided to then use it on us. In hope to create a new Organism.