Premium Economy won't be of help in none of those airports, since there are no priority security lanes in GRU/AEP/SCL or CJC.
You will have access to priority check-in in all of them.
Catering will be non-acoholic drinks plus chips/crisps/sandwich/cheeseburger depending on length and time of day.
Your back will be labeled all the way to CJC on AEP but you will need to retrieve it in SCL and recheck it in the counter, pretty much as it happens in the US