I think at the end, he was so convinced that wherever he was (dream or not) was real enough for him to stay, so he went there. Notice that his kids never aged throughout the movie, which makes me think he still may have been in a dream.
I also didn't know how the dreams were actually shared. The machine was only for keeping them in the state, but their brainwaves weren't being correlated/captured/compounded into one from anything they showed us.
In terms of the last dream being shortest, that was the limbo dream. That was an indefinite time frame, which just happened to be short for Leo, not for the other CEO (forget his name), which they allude to right at the beginning. The "kick" that they kept showing once they got into the third layer was so scattered, it was hard to really grasp how much time each "action" took in each level. The van falling was barely shown for any length of time, and was slowed down, so it's hard to say. The elevator kick wasn't as bad, because he was able to complete it without distraction, but without gravity.
Overall, a great concept for a film. Very easy to follow if you pay attention, but I'm sure I'd catch more seeing it a second time. Very loose notions of the first "Matrix" IMO, but not enough to feel like copying it. Nolan is a genius for designing scenes that other directors would simply use CGI to develop. To me, he's a modern nod back to the golden ages of film making.