what came first, the lifeform or the goo?

First of all i would say that to me it would seem (things could be a bit muddy with AC) that the Sacrificial Goo and Black Goo are not the same but the Black Goo is the result of the Sacrificial Goo.

Sacrificial Goo => Breaks down DNA of Ingesting Organism

The resulting broken down Genetic Material is then spread into the Air/Waters and infects Basic Organisms to Evolve them and pass on DNA Traits of the Organism that consumed the Sacrificial Goo.

Spaights draft is the easiest way to see this Process as Goo is replaced by Nano-Scarabs that consume molecule by molecule the Engineer and then one of these Scarabs then encounters and bites a Female Primate and Injects the Engineer DNA which then causes the Primate to Evolve/Mutate into a Engineer/Primate Hybrid.... Thus => Mankind.

If we look at the Black Goo...

Black Goo => Hybrid Xenomorphic Strain DNA Organisms

Again Spaights draft gives us the better indication, as the Urns in his Draft contain Nano-Scarabs which instead of breaking down Fifield... they just attacked him and Injected him with a Alien DNA and he mutated and Evolved into a Xeno/Human Hybrid with Acid Blood.

Which then when we look at the Sacrificial Mural, shows a Xenomorph like Organism in Cruciform Pose (represents Sacrifice) the Sacrificed Engineer in Spaights draft took the same Cruciform pose before his body was digested.

In front of the Mural was Originally the Sacrificial Cup

So it is likely that Originally and its always been my interpretation that these Engineers had came into contact with a Organism related to the Xenomorph, that infected a Engineer to Produce the Fresco Organism.

They then began experimenting on this Organism, and modifying it until they achieved what they had in the Mural, and at this point they saw this as Perfection and decided to Sacrifice it to produce the Black Goo that they now saw as the more PERFECT Seeding DNA as opposed to Engineers DNA.

So yes i think they had encountered a Alien Life-Form that the Xenomorph Strain had Originated from.... Mankind came from a Goo Strain Obtained from the Sacrifice of Engineers, that was further evolved upon and experimented upon until we get Modern Mankind.

And so likewise the Organism these Engineers encountered was Sacrificed and experimented on over and over until the Xenomorph is a result... which now seems to be  created by David.

As for the Scorpion Ship

This is a interesting part of the movie, did it crash? did it contribute to the Juggernaught crashing?  Was it manned or unmanned?  Did it leave Paradise/Planet 4

This is so ambiguous that it will mean a lot of debates.. i for one think this is a Watchers Craft of the Elders/Engineers and that the Elders or Engineers are the Cherubim of the Bible/Paradise Lost

The LV-223 Engineers are a rebellious Faction Banished from Paradise, that was either once these Watchers/Cherubim or those beings on Paradise or their Predecessors.

Who in either way... had Rebelled against their Purpose.

Great topic !!

My favourite character in the franchise is this creature (in fresco) who seems to be "under" the Engineers, almost child like pose, very submissive.

The bird faced character - I sometimes like to think (yes im that sad!!!!!) that perhaps this bird faced creature is somehow involved with the black goo.  Does it have this stuff inside its body and its where the goo originated.  Possibly on its homeworld.

I used to think it may have been involved in creating the first egg. There is another pic similar holding an egg, Big Dave knows it and it looks a similar creature from what we see.

Either way for some strange reason i love the characters we have seen the least of.   This creature and the elders are special to me.  So many possibilities.

How i hoped that this creature got a mention in a:c

I believe the bird figure is simply related to the Prometheus figure, and nothing more. Of course it does have biomechanical features like a xenomorph ..