Thanks to Treker4747
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).
Let me understand the totem piece further. What is so special about the top that it is Mal's totem? (and later Cobb's)? Everybody in the dream world knows (assuming projections are rational) that a top on spinning shouldn't continue spinning but eventually come to a stop. So - why would the top keep spinning if a projection models it? A car doesn't keep running cos projections that cars normally stop when people use brakes! Similarly why wouldn't a projection know that a top is supposed to spin only for a short duration and then topple over?