I think I understood most of the story regarding Fischer, but there were some points I didn't quite get regarding Cobb and Mal.
Cobb knew the "inception" could be done because he had done it to Mal, his wife, when they were in limbo. This was because Mal had started to believe limbo was real. Cobb planted the idea that the world they live in was not real and that they had to kill themselves to escape this world. Unfortunately, when the two woke up, Mal continued to believe her world was not real and killed herself in reality.
A. Is this correct? If so, isn't the idea that Cobb planted actually the truth? They WERE in a fake world and killing themselves was necessary to escape it. Mal just couldn't recognize it anymore. Is that still considered "inception" if the planted information is real?
Yes, Cobb planted the truth. You can plant truths , it doesn't need to be false information.
B. What exactly was the purpose of Cobb breaking into Mal's safe (in the dollhouse)? He broke in, picked up the top and spun it.
Mal's safe was a corner of her mind where the top stopped spinning, even though it was supposed to continue spinning because the reality was in fact not real.
Cobb started it spinning again.
Now Mal looks at her totem in the corner of her mind and realizes that this reality was not real.
It's assumed that Mal obviously knew Limbo was not reality *in the beginning*, but eventually Mal wanted to stay in Limbo and convinced herself that it was reality. Thus, why Cobb had to perform an inception on her to break her own self delusion.
C. When in the limbo state, Cobb and Mal eventually woke up by lying down on the train tracks, correct? Why were they still young looking when they were on the tracks? They spent 50 years together in limbo and a scene showed two old folks walking. But Cobb and Mal were still youthful when they lied down on the tracks.
That's either a discrepency by the producers, or just artistic license because Cobb is telling the story. He told the story of when they grew old (using the old images) and then telling the story of when they got back to reality (by improperly using youthful images).
D. Limbo is a state when you've "died" in your dreams, but too sedated to wake up. How does killing yourself in limbo help? Cobb and Mal lied down on the tracks, Saito and Cobb (presumably) shot themselves to wake up from limbo. If killing yourself wakes you up from limbo, why not kill yourself sooner?
Again, a good point. Actually, I don't think it's killing yourself that matters, it's the "kick" that gets you out of Limbo.
Remember Ariadne jumped off the building and the falling sensation was what kicked her back to Level 3, indicating that they just needed a strong kick to escape Limbo.
But of course, that doesn't really explain why the whole story ran with the assumption that Limbo was so dreaded. It seemed like it was easy enough to get out of.