Thanks to Treker4747 and Ryan for the answers to my questions.
Let me see if I understand correctly. Limbo is equivalent to the 4th level of a dream? (1st level - Van, second level - hotel, third level - James Bond Ice Fortress, 4th level - Limbo)
Ok - also too much time in Limbo will mean your brain turns to mush. But Saito does spend too much time there. (He's a really really old dude when Cobb finds him) So how come when he wakes up, he's alert enough to use the phone and do the plot-ex-machina to make sure that Customs will allow Cobb tome thru (the charges against Cobb are removed from the official databases based on his interevention). (Btw, that answers the question of how Cobb is able to get thru US Immigration & Customs without problems. (Or he could be using a fake identity but then why didn't he try that instead of staying out of the US and not meeting his children).
We don't actually know how much time in Limbo is "too much time." He was a really old man, yes, but as soon as Cobb showed up, he remembered who he was and what he was waiting for. The fact that they managed to keep Saito alive through the other dream levels for as long as they did probably helped.