A couple of questions:
* I know that traffic in SP is pretty bad, but on a Saturday morning, how long do you think it would take for me to get from GRU to Pao De Queijo Haddock Lobo in Jardins? Also, how long will it take to get from the Fair to Mocoto Esquina?
GRU to Jardins (Haddock Lobo) on a Saturday morning should take you around 45 minutes. Not much traffic expected that early in the morning.
From Benedito Calixto (which does really get going in the PM btw) to Mocoto should take you about 50 minutes - do keep in mind you should get there just shy of midday to avoid horrendous queue....
* Is walking from Pao de Queijo Haddock Lobo to Fair Benedito Calixto safe? It looks like the walk is about a mile and looking at the google street view, appears to be a fairly populated area.
Fairly safe during the day, but you can be the judge of that. Uber is super easy and cheap in Brazil if you do change your mind
* Would it be best to leave my day pack at the airport?
I would assume so - just so you can walk around better. It should be easy and inexpensive to do that at GRU
* I'm assuming that 2.5 hours would be enough time at the airport in the afternoon? Mocoto Esquina is fairly close to the airport so I'm figuring that as long as lunch was over by 2pm, I would be fine? Yes, that would be plenty of time. Just bear in mind that you do need to get to Mocoto fairly early to be able to dine in peace....
* Do most places take credit cards or would I be better served exchanging USD to Reals when I land?
Most places most definetely do take credit cards - no problems there. Do note that the most common credit cards in Brazil are Mastercard and Visa. If you have any of those, then you should be safe.......