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).
The "brain turns to mush" thing is over stated. It's more that the more time you spend in Limbo the further away the "real world" is. Cobb and Mal spent something like 40 or 50 years in Limbo a long time, for sure, but hardly such a long time that they would've forgotten their real lives beyond just being, as Cobb puts it, "old souls." Cobb and Mal, under a standard sedative, would've only been a sleep for something like 8-10 hours for the 50 years to pass "in Limbo."
The problem with the final mission was that the sedative was going to keep them asleep for 10 hours and it made the time dilation that much more powerful, it increased it to factors of 20 meaning the 10-hour flight would equate to almost 300
years in Limbo. The idea was that Wantabe would be "so old" by the time the flight ended that he'd have forgotten his promise and even who he was. Moreso, the idea was that anyone
would experience this but that idea seemed more theory as no one had gone that deep before aside from Cobb and Mal. Wantabe, however, said he'd remember the promise and seeing Cobb helped him remeber that promise so was able to remember to keep his promise and still be, somewhat, cognizant when he woke up, though it'd likely take him time to readjust to his life. Again, think of the TNG epiosde "The Inner Light" for a similar idea to this.
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?
I think it might've been less "the top won't fall due to lack of gravity in the dreams" and more "that's how I want my totem to work."
I mean if it was the gravity thing that's apply to a lot
of things we see occur in the movie. But since Leo's idea was for his totem to spin "forever" in his dream that's how it works not because it can't
fall over but because Cobb, subconciously, won't let
it fall over. This is probably where the rule of not letting others know too much about your totem comes from as it creates potential for abuse. Mal had, apparently, let Cobb know how her totem worked so he knew how to exploit it to manipulate her. So even if she couldn't "physicaly see" the totem spinning/not spinning somewhere in her mind she knew what it was doing even on a subconcious to her subconcious level.