SPOILER!
 
Okay, so Nicole was never actually there. But at one point in the game, Isaac has to defend her from monsters and she opens a door for him. How did the door really open? Who or what actually pushed the button?  I never got that. The vision of Nicole physically interacted with the environment. What's the deal with that sequence? Does the marker create solid visions?

Maby the door was open the whole time and Isaac was just hallucinating that the door was closed.
And remember, isnt it Nicole who jumps him at the very end when he's escaping the planet?

"Maby the door was open the whole time and Isaac was just hallucinating that the door was closed. And remember, isnt it Nicole who jumps him at the very end when he's escaping the planet? "

" Maby the door was open the whole time and Isaac was just hallucinating that the door was closed. And remember, isnt it Nicole who jumps him at the very end when he's escaping the planet? "

Wow, this same discussion was had a little while ago. 
 
Basicly, a bunch of people came to a bunch of ridiculous conclusions, but Achems razor dictates the simplest is the most likely, and imo the simplest solution (and the most likely in my own opinion as well) was that he simply halucinated before shooting the junction box where she was supposedly stood. Or he stood there halucinating until the door malfunctioned on its own. Or as has been suggested, it was already open. 
 
Basicly, the simplest answer is it was a simple halucination. Given the ending, and EA's confirmation that Isaac is still alive, i'd say that further backs up the idea it was a halucination. 
 
As for your theory on the ending, no, it's a simple plot device designed to simultaneously set the game up for a sequel, and wrap it up incase it doesn't get one. With that one ending, you can make a sequel and say it was a halucination and Isaac is still alive (or it wasn't, and he miraculously fought her off), whereas if the suits don't sign you for a sequel, you say he died and it was real. Simple. It's not 'outside the main narrative' you just aren't told which of the two possibilities is true. Although given they confirmed he's still alive for the sequel, i'm guessing it was option A, halucination.

The_A-Drain makes sense with his explanation of the second ending.
 
I think I heard a sequel had been approved. That's awesome. 
 
At first, I thought Nicole was real and there was a chance for Isaac to rescue her but by mid-game I had a strong feeling she was gone. Very sad.

I just finished this game and man, I did not see this coming at all.  I believe that the door was open or malfunctioning at the time, and Isaac hallucinated Nicole there until the door opened.  
 
And I agree with A Drain, the last scene's purpose was to either wrap-up the storyline or bridge to the sequel.  With Dead Space 2 basically a sure thing, I believe "Necrole" won't be a hallucination, but Isaac will finish her off.  He still had his suit on and he had just fought many things tougher than her walking down a hallway.  Also, his tools couldn't be that far away.  It would be a shame for him to die after all that and kinda like a bitch, relatively speaking.

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Haha! Now we have these! Ah, I can't get enough of them. Time to stop abusing them. But after this one.

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" @Strongba1 said:

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Haha! Now we have these! Ah, I can't get enough of them. Time to stop abusing them. But after this one.

"

How do you use these? "