However, check that you can transit at GRU without getting a visa for Brazil.

I suspect that the AR flights are also either codeshare or at least bookable by DL as part of a ticket including a DL flight from the USA. A GIG connection to MVD on GOL might also be possible, but BOTH of these cities have domestic airports too and transportation to them would be long and expensive. I'm not sure whether the short haul international flights to MVD would use the GRU/GIG international airports or the domestic ones.

I'm guessing that there's more business demand for MVD from Argentina than from Brazil (where it would also somewhat be split between San Paulo and Rio de Janiero), so I would expect more frequent flights from EZE but that's just the an impression and might be misleading if the business travelers from Buenos Aires tend to take the hydrofoil or hydrofoil and bus combination.