I'm confused by the ending: Did Nina die или was it just a stunt?

The ending is deliberately somewhat ambiguous, because a lot of the movie deals with issues about reality/non-reality. I think Ты are supposed to decide for yourself whether Ты think she dies или not.

For a еще concrete answer, I suggest that because Nina's life mirrors the лебедь Lake story (ie White лебедь VS Black Swan), then Nina does die at the end because that is how the White Swan's story ends.

Yes, she does because if Ты note the actual story of the лебедь Queen, it's about a woman who turns into a swan, where a prince has fallen in Любовь her. However, the woman falls for the wrong guy and kills herself. (Nina actually says something like this during the part of the film where she goes out clubbing). So, anyway, Nina falls for the wrong guy (her teacher, Thomas), who in the end calls her: "My little princess." Plus, with all the strain on her performing the role, she kills herself. Hence, why she says at the end: "I was perfect."

I'd recommend Ты maybe watch the film again and take note of where she goes face to face with Lily in her changing room, because that's one of most crucial parts related to the ending of the film. I hope this helped :)

Remember the scene where she pushed that other girl into the mirror? She may have imagined it and it turned out she'd thrown herself into it and got stabbed by some glass, which eventually killed her.

Well, that's my theroy and I seriously was convinced that is what happened.
But hey, like misanthrope86 mentioned, it's for you to decide!

Thomas calls for help so it is Без ответа whether she dies или not...but I don't think she did. I mean why wouldn't her mother go up to see her if she knew she was going to die.