For your return flights, in EZE, you should receive both boarding passes and your bags should be checked all the way to DFW. In ATL, you pick-up your bags, clear customs and immigration, recheck your bags, go through security, then proceed to your next gate (or the lounge). There is an area to re-check bags and clear security next to customs/immigration - sort of like the "transfer area" in LAX except that everyone goes through it, even if their trip ends in ATL. If for some reason you don't get your ATL-DFW boarding pass in EZE, there are kiosks and rebooking counters in the secure area of the airport where you can get your boarding pass after reclearing security.

I'm not sure how busy customs/immigration will be (there are several early morning flights and it depends on if you beat them or arrive after them), but with 6 hours before your connecting flight, you should have more than enough time.

Let me add, I'm a bit jealous of your EZE trip! It's a great city to visit. Hope you have a good time.