Qatar states they interline with all carriers (but only if issued on one ticket I think): http://support.qatarairways.com/entr...line-bookings-

Checking in at EZE might create some issues as I presume they outsource the check-in. Call Qatar to find out and have them make a note in your PNR that you are connecting to xxx etc. and checking through luggage. You still need to be prepared for it not to work. You don't need a visa to transit in GRU. I would go with the carry on - easy enough for 3 weeks.