Q & A: "Ariadne's Paris training sequence"
Q5.1) Why was Ariadne stunned when Cobb told her she was in a dream?
Answer: It was only her first training session. Thus, she was unable to differentiate between a dream and reality.
Q5.2) Whose dream was this sequence based on?
Answer: Cobb’s dream. My understanding is this: Cobb was an architect before, and had designed this level to train new recruits. He invited Ariadne’s subconscious into his dream to teach her the nuances of designing a dream. Bystanders who would stare at Ariadne were the projections of Cobb’s subconscious, and they did so because she was a stranger to them, and she had changed some of the architecture of the dream.
Q5.3) If it was Cobb’s dream, how could Ariadne change the dream’s architecture e.g. creating a bridge?
Answer: Ariadne could change architecture because she was an architect herself, and it was her training session.
Q5.4) Why did Mal suddenly appear and stab Ariadne?
Answer: Two reasons. One, we learnt that Cobb had been unable to control his subconscious projection of his dead wife (who would repeatedly haunt him in his dreams), and it was a condition that would get worse later on. Two, Ariadne created a bridge out of her memory, which Cobb recollected that he used to spend time with Mal there. Cobb then warned Ariadne that creating something out of memories would bring out sides of the subconscious that they would not like to see.