I am a DL PM and flew JFK-CUN last fall (DM-coded, AM-operated)
I got upgraded both ways, but the IT linkage between DL and AM was messed up. The reservation got weird after it tried to upgrade me, I spent an hour or so on the phone before the outbound (don't remember about the return).
The AM airport employees understood how upgrades are supposed to work and were helpful. They processed the upgrade on the outbound; don't remember if I needed their help on the return.
Not sure about intra-Mexico, or Mexico-Central/South America flights.
I got lounge access in JFK and CUN without a problem.