Is it all on the same ticket - try get as much confirmation beforehand. All I will say is if they can't check your bag and you into all your flights in EZE then don't go because in my experience there is 0 chance of anyone entering Brazil to collect bags or check-in that is not a resident or Brazilian citizen. 30 hours in GRU - ouch... Everything is pretty much closed on the airside except 1 cafe which has some food. No lounges are open either.
You improve your chances of having no issues with missing bags if you can get away with no checked luggage. Just make sure you check-in to the United flight BEFORE you leave Argentina and have the boarding pass on paper or your phone because you will need it to enter airside terminal of GRU when you land and I didn't see any 'transit' counter in GRU before going through security to enter airside.
Ahh ok sorry I misread your post. Are you in contact with the Argentina embassy in USA? Is the United flight an official repatriation flight - if it isn't then you will have a problem because you are required to do a medical check prior to flying to EZE (this is normally done at check-in and organised by Embassy).
OK good - I ended up doing it all on my own and had to last minute try convince the Argentine embassy in Sao Paulo to come airside and do the medical check there since it was being done at check-in. Luckily they agreed or I would have been on a plane back to Europe. I would double check with the embassy and confirm where the medical check will be taking place.
So, I was finally able to leave the country on last Thursday, took a cabify (taxi app) to the airport and carried a printed version of my ticket, the printed letter from the ambassador and passport in hand We zipped through the city quickly , we weren't stopped at all by police (as they say it would happen) and it arrived at Ezeiza Int'l airport 7 hours before departure (I really didn't want to miss it) , the airport had a weird atmosphere, all the shops and restaurants were shut, except McDonalds who was serving meals/coffee, most of the screens weren't announcing departures (only two commercial flights were leaving that evening) , so you kinda have to ask around for info. In "Migraciones" / customs they make you sign a document that you won't come back during the pandemic , I eyerolled a little ... when I arrived in Spain 12hours later I immediatly could feel a more relaxed vibes / less restrictions , hoping to come back to Argentina, when things are better.
