Too many folk to reply to all at once (or too much effort to quote . . . :) )
So.
The mag does say they are engineers. Read it! That could mean anything though at this point as it also says they are a civilisation, rather than a race, so diversity may be common? We just don't know enough yet.
I'd also wager they don't look so peachy after x amount of years in some form of cryo sleep away from sunlight. We'd look pale(r), no? Possible reason, anyway.
As for the eyes, maybe another adaptation to stasis/space travel? Who knows but those 'suits' look to merge with biology so who knows what effects they have. The 'sacrificial' fella in Prom would surely have just been wakened from a long period, no?
The engineers we see in Prom could be clones (but I doubt it) or simply a slightly different humanoid type that is part of the whole engineer culture? Are our astronauts reflective of the human race en masse? I'd say they show the very pinnacle of our species (subjective, I know) so perhaps the same occurs with Engineers?
With regards to technology why assume because they don't all carry iphones and aren't whizzing about on hover boards that they must be less advanced? The ship says a lot (we can't do that!) We also only see a tiny portion of the whole world so who knows what else is there . . .
It also says that they discovered a kind of way to manipulate gravity that we haven't realised yet, so perhaps they have some huge advances over us but also lag behind in other regards? It's possible. Or maybe their knowledge/tech prefers a more natural aesthetic? If you are advanced maybe you favour or see the need for more 'natural' technology? Are cars good for Earth, as an example?
If they saw the juggernaught approaching we don't know what they would think. Welcome home, who's this, that's odd etc . . . I doubt they'd suspect an AI humanoid from Earth with an apparent need for revenge, carrying a whole ship load of the bad kind of gooeyness, was about to start the party . . .
I think some folk here are being either a bit too simplistic or massively over thinking things :) (there's irony for you)
I think the big question(s) here now is why has David done this and where is Shaw? Is this the first place they've visited since LV223?
My feeling is that this is a civilisation that is part of the whole engineer culture. Perhaps multitudes of humanoids that vary slightly all form a larger culture. This could be where the idea of civil war and abuse of the 'sacrificial goo' may fit in?
What also interests me is if there is a race or being above them (engineers)? Perhaps the original 'humanoid', as we know they seed life elsewhere, or even something else entirely?
Disappointed to see David being the 'destroyer of worlds' in some ways but if the above is somewhere near the answer then it opens up many more doors and intrigue . . . Sets up and adds more clues to the enginners' culture and links to ALIEN (which is what I'm mostly here for, mostly)
As ever, it's certainly going to look stunning if nothing else :)