I'm a newbie here and a sometime lurker. I searched for the topic before, but I couldn't find what I needed, perhaps because my questions are a bit obvious.

My girlfriend (not a Brazilian citizen, but she doesn't need a visa) and I (Brazilian) are going to Brazil to visit family in March. Family is outside Rio (GIG, SDU), in Juiz de Fora (JDF, or bus from outside Rio), and outside São Paulo (VCP or GRU are most convenient). Whereas we will stay a few days both around Rio and in JF, we are not planning to spend more than a few hours with family in SP State.

I understand we could fly JFK-GIG and then do something like AD SDU-VCP (or go by bus) and P8 or T4 GRU-JDF, but when searching for our JFK-GIG flight (in Y), JJ gives an option with a roughly 12-hour layover in GRU that would give us enough time to make the ride to visit the part of my family outside SP. First flight is JJ 8081 and arrives in the early morning, and the second flight (I don't remember the number now), GRU-GIG, leaves around 7PM. Sounds good to me.

Now, my questions are the following:

1) Luggage: If we check in luggage, what happens to it once it arrives in GRU? Do we have to pick it up and recheck it for the GIG flight just because the layover is so long? Having flown with JJ before, I know this probably doesn't happen, so does the luggage fly with us on the same GRU-GIG flight, or does it go to GIG asap?

2) Customs & Immigration: I assume we can just leave the terminal at GRU and reenter in the late afternoon, particularly since we will get the BPs for the GRU-GIG flight at JFK. But, in all the times I flew JJ with a layover in GRU, I always went through customs & immigration at GIG. So, what happens when we arrive in GIG, since my girlfriend will have an entry stamp on her passport from a few hours before on that same day?

3) Rescheduling the flight: Is there a significant chance that JJ would decide to reschedule us for an earlier flight because of the long layover, either beforehand or at check-in at JFK? I guess I could just refuse the offer at check-in.

Thanks all for your input and advice!