I can't believe I just saw this film. Totally amazing!
Firstly, I think Leo was incepted by Miles (Michael Caine) and Ariadne.
Cobb tells Miles that he has to do one last job to get himself back to the US, since Saito has offered to help him if he's successful. Miles, on the other hand, knows how to get Cobb to the US without him
ever needing to do anything stupid........get him to complete the job in his own mind, and then let him see his own kids in Limbo. Cobbs will finally have peace.
At some point, Cobb is put into a dream. This probably happens under one of Yusuf's drugs, when they're planning the heist at their headquarters, and Ariadne was around to do the dirty work for Miles. Cobb never really wakes up, and is dreaming from that point forward. Ariadne helps Miles "incept" Cobb by essentially playing a psychiatrist for Cobb so that he can go through these scenes in Cobb's mind "elevator" again, but viewed from a more objective point of view (Ariadne's POV).
Why do I think this? Well, the dreamer of each level needs to stay behind in order for everyone else to delve further into another dream. Yusuf has the 1st dream, and stays behind to drive the truck (and instigate "the kick") while everyone else goes to the hotel. In the hotel, Joseph Gordon-Levitt's character is the one who's having the dream, so he has to stay behind and instigate the kick. Who's dream was the 3rd? It was Fischer's dream. However, when the kick worked, and Fischer was out of there, how the hell did Leo get to stay and enter Saito's dream while Fischer was already gone? The dreamer needs to be in his own dream. Otherwise, there is no environment!! That's because the "rules" I mentioned before are, while true in reality, do not apply when the entire plot is actually happening in Cobb's mind! All the levels are Cobb's, and the only other person in the film who's real is Ariadne.
Near the beginning, when Saito was in the helicopter (which happens in reality), he said something to Cobb like, "Do you want to become an old man, filled with regret?" Uh.....the answer is obviously "No thanks". However, that quote lingers in Cobb's mind, even in his dreams, so that even in a dream state, where Cobb is planning the inception on Fischer (while in that dream), all the events are just ways to jog his memory so that he realises that Mal's death does not mean that Cobb broke his promise to her that they would grow old together. They did grow old together.......when they were in Limbo. It just seems like the memory was buried deep amongst his own guilt.
Anyway, when Cobb confronts Mal in the Level 4 dream, near the end of the movie, he tells her that they did grow old together, and he had kept his promise. He asks her if she remembers this. Of course, Mal isn't really there, and Cobb is just struggling with his own memories, trying to overpower his negative feelings (i.e. his guilt) with a positive thought that while she did kill herself, he shouldn't feel continuous guilt for a broken promise.
He always knew they grew old together, but apparently, this memory was so well buried inside his mind that he had to delve many levels deep for him to convince himself that he could move on.
Now, Saito did offer Cobb a ticket home if he was successful, but since Cobb told Miles about this, and Miles had essentially incepted Cobb, the only way Miles could honour the agreement (since the real Saito wouldn't have due to him not completing the job), was to put Cobb into limbo again, but one where he had his children.