Why in the prologue people are astonished by "floating rocks"? Why do they point it out?

As I have stated over and over, its not the fact that they explain the scourge moves later in the game.  Its the fact that the image at the beginning is that the ship hit it, despite seeming to have plenty of time for them to avoid it.   By the time they "hit" the scourge, there is bridge crew, numerous engineers, medical staff, Ryder is awake, has timeto drink a cup of coffee, and THEN they hit the scourge.   At that point, there is no indication it moves.   This could have easily been explained right then if someone commented, "Holy Crap!  Did you see how it moved?"   Or have them hit it immediately on arrival.  But instead, I (because i didnt read the book and have any knowledge of the scourge at that point), am left to look out the window at a space sponge that takes up half the cluster and wonder, "WTF?  How did someone not see that and avoid it?"  There are no "blatant details or plot points" AT THAT POINT in the game.

Yes, a few hours of gameplay later, you get more insight.  But AT THAT MOMENT, you are left to wonder WTF.  

When you get to the bridge, the Captain could have just said, "That thing moved towards us!". Problem solved.

If you've already taken a few breaths of who knows what gases that make up Habitat 7's atmosphere then passing out quickly - already little oxygen in the lungs (because you've breathed out) and a lot of other stuff that might not do you much good at all - is quite likely. Even if the pressure was low the air won't be "sucked out" unless you breathe it out. Being in a very low pressure environment, which they weren't, will do all sorts of unpleasant things to you. That isn't one of them.

They could only live by passing the helmet if both remain concious. Ryder Junior passed out even with the helmet on.

What was in it? It's been a while since I played through the prologue. IIRC Alec talks a bit after taking the helmet off. If they'd analysed the atmosphere "dangerously low pressure" would've been one of the first things noticed (and would make the plant life unlikely) - besides, 2.3 atmospheres.

Are folks arguing physics in a Mass Effect game? It's the home of resurrected Shepard's and Space Magick. It just is what it is.

That's complete speculation on your part. You have absolutely NO idea how a human body would react to breathing an atmosphere of that composition. It takes a very special mixture to sustain us, and too much of anything EXCEPT that builds to TOXIC levels in your blood stream. Also given that it takes very little time to lose consciousness without an adequate composition to breathe, unless he was holding his breath to begin with he was doomed on his first exhalation and he is rapidly breathing, RAPIDLY, accelerating this process.

Again, the pressure is greater and his lungs are not decompressing (because he is drawing breath and speaking.) Experiencing a vacuum would have emptied his lungs completely and he would have been able to respirate AT ALL, not JUST the oxygen. (Again, not even 21% of our air composition.)

Ryder jr WAS still conscious when the helmet was placed on him. Unfortunately having his air supply restored did NOTHING to restore his consciousness, ergo damage had been done besides simply not having oxygen. (Like several lung fulls of inhospitable atmosphere.) 

And Alec would have passed out very soon after passing his helmet to Ryder junior because HE'S BREATHING AIR THAT IS KILLING HIM. In absolutely NO way is anything being sucked out of his lungs, that would interrupt and HALT respiration if that were the case.

Lastly, if the air HAD been sucked out of his lungs he more than likely would have stayed conscious LONGER than if he'd been replacing the oxygen in his blood stream with completely incompatible elements. Respiration would ACCELERATE the process of expunging the oxygen from his body, rather than just removing the single breath from his lungs. (One full breath can sustain the brain and its activity for CPR.)

Either way its all moot. Habitat 7=2.3 atmospheres. Case closed. No pressure variance for the "oxygen" (and only the oxygen huh? Because that's how the physics of it work....riiiiiight) to displace and pull the oxygen (again ONLY the oxygen.... Huh. Weird.) from his lungs.

The exact composition wouldn't really matter. If it were anything OTHER than a breathable mixture, inhaling and exhaling would accelerate the asphyxiation process. If the pressure was low (though unless another source could be found, 2.3 should be in comparison to Earth as a base.) Alec and Ryder are showing zero signs of depressurization. (Oxygen, as well as any other gasses, liquids would have been expelled to balance the pressure. Not JUST from their lungs, which again isn't happening either as their lungs are able to function.... Unless a lung full of air were able to balance out the pressure for the entire planet in comparison to their bodies.)

And I'll also give you that as well. But we can at least say ME1 was close to the realm of implausible and me2 and later is the realm of impossible.

I'm with you. The whole time I sat there and kept saying, "This is the equivalent of an iceberg, but they can clearly see it. Where was everyone looking?" I started to get annoyed and kept thinking to myself, what kind of crew is this incompetent? What are the authors trying to tell me here? Yeah... it made sense later, but I was already internally aggravated. I started noticing little details that weren't mentioned or off from there on. Nitpicking for lore and canon. My OCD kicked in.

No, world building was in ME2. That was it's strongest part of the plot. Physic on the other hand went totally out of the window in to the dumpster.

I don't remember that Eden Prime was terraformed at all, it was just a world they found and colonized. 

I don't remember terraforming being a factor at all in the first three games, but if someone can dig something up to that effect that's fine. 

I made that assumption. Jenkins talked about Eden Prime. Nihlus compliments Shepard about Eden Prime. I just made the assumption. I can't find anything on the wiki to support that. 

UPDATE EDIT: I'm replaying ME 1 and I made all sorts of assumptions. I thought because if it already was a garden world, why isn't anyone colonizing it? I made the assumption it was some planet that needed terraforming near the frontier of Citadel space. However, after replaying it Jenkins is stating how well the colonists are keeping the planet clean and as pristine as possible. The planet, if it was like this, the Batarians would already own it. Guaranteed. Nostalgia is a m*****f******.