Well they said themselves that they were 3 and five years old during the telephone conversation. But I can't quite remember if their conversation on the phone was a dream sequence or not, but it's most likely not IMHO.

Does Cobb’s totem keep spinning at the end or is it about to fall off the table?
A: The fact that the film cuts away before we know for certain suggests that they want us to keep guessing. But we think it kept spinning. Here’s our reasoning: Note that at the end of the film Cobb’s kids haven’t aged. They match exactly his memory of them. A memory which must almost surely by now be out of date, since he’s been away from them for many months. Though he finally sees their faces, otherwise they look exactly as he envisioned them. They're even wearing the same clothes. In reality, his kids would now be older and different than his memories of them. This could suggest that Cobb is still in the dream and the top does indeed keep spinning after the credits roll.

Alternate Theory Aaron points this out in our comments section: "In the opening moments you get a glimpse of Leo's hand. Specifically, he's wearing his wedding ring. Now, if you follow the rest of the movie keeping an eye out for this you will notice that he only has the ring on when he's in the dream world. At the end of the movie he isn't wearing the ring." If the ring only appears when he's in a dream and he's not wearing at the end of the film, that could be confirmation that in fact, the top does stop spinning after the credits and Cobb is at last in the real world.

1. & 2.
I think there was a dream device attached to both Cobb and Mal. That's the only way to share a dream. I also believe they used heavy sedatives. Cobb did say they were exploring deeper levels in the dream world.

Here's what I think. Many levels into the dream world, time became extremely slow. In the last dream level they dared to venture in, they basically spent 50 years there, aged, Mal went into a limbo state of mind, Cobb kept his sanity, decided it was enough, and killed themselves. Note that this particular suicide was not shown in the film. They then traversed across the preceding levels, killing themselves in each of them so they could go one level up. Cobb kept count of the number of levels they were in. The suicide on the tracks we see in the film happens when they reach the first level, thus explaining why they were still young when they lay on the tracks. They both wake up in the real world, but Mal, infected by Cobb's inception, refuses to believe this is reality, instead believing she will (always) need to go one more level up to reach reality.

3. True, Mal locked her totem in the safe so she could accept the dream as reality. Cobb took the totem and made it spin indefinitely. Presumably, when Mal opened the safe and saw the indefinite spin, she realized she was still in the dream world. This was Cobb's first inception.

4. When they died on the tracks, they woke up in reality. Mal constantly appears as a malevolent projection of Cobb because Cobb is haunted by his guilt of causing her death, and also because he misses her so much he simply cannot keep her out of his mind. Essentially this means his projection of Mal has become something out of his control. The various "levels of Mal" Ariadne secretly sneaks into are really just memories of Cobb together with Mal. He keeps these memories because this is how he gets to be together with her.

5. This question pertains to the concept of limbo, which I'm really not sure of. The film says Cobb was in limbo before, but as far as I know, only Mal was in limbo. Cobb was always aware he was in a dream. Perhaps by having a dreamer (Mal) in a state of limbo, and by sharing the same dream space, Cobb shares the same limbo as that dreamer, although as said he was always aware it was a dream. Limbo is said to be a vast empty space of nothingness in which the dreamer-in-limbo perceives as reality. Things only appear in it when the dreamer starts building them, or if someone who has been to limbo occupies the same space, which then fills the limbo with buildings and projections from the old limbo. So when Fischer enters limbo, that limbo is empty. But when Cobb and Ariadne enters his limbo, Cobb who has shared limbo with Mal, fills the space with his old projections.

6. Basically, the inception was done by seeding doubt in Fischer's mind that Browning was trying to steal Fischer Sr.'s will that states Jr. will have the choice to break up Sr's empire and do what he wants. This was actually done in level 1 (the hostage scene), where Eames, impersonating Browning, tells him how much his had loves him and makes it known to him that he has a locked will somewhere, strictly for Fischer Jr. The seed is sown, and from then on, even if Fischer himself doesn't realize it, he subsubsubconsiously (lol) thinks his dad loved him and was only disappointed that he tried to follow in his footsteps. In Level 2 (hotel), the Browning that enters the room is actually FIscher's projection of Browning, and Browning confessing to trying to steal and destroy the will in the safe (so Fischer wouldnt break up the empire and Browning could impose his influence on the company) was actually completely simulated by Fischer's own subsubsubsubsubsubsconcious mind (lolol). It was actually done in a very subtle manner. In Level 3, basically his subsubsubconsious mind makes up a projection of his dad telling him that he didn't want him following in his footsteps, and this the full idea of dissolving the company is implanted. I think that's how it goes. It sounds a bit silly, but I think it's quite an original idea.

7. Basically both Saito and Cobb were stuck in limbo. This time Cobb really IS stuck in HIS own limbo, meaning he doesn't know it's all a dream. When both of them come face to face though, each reminds the other of something that makes both of them realize the world is a dream. Saito sees the totem and is reminded of Cobb, and the whole "die as an old man, full of regret" dialogue reminds both of the agreement they made in the real world. I would like to think Cobb managed to realize this because of his clinging, parasitic desire (or idea, if you will) to see his children, and Saito because of his Japanese tradition of honoring agreements?