I’ve done it several times. It’s no problem. Don’t underestimate how badly it sucks to be up on the flight deck all night long though, while the other 3 are doing their break rotations, and have the lights on trying to stay awake. You won’t be doing much sleeping.
It helps to make an effort to properly introduce yourself to the purser as well - don’t do it as an afterthought like typically happens domestically. He/she may take pity on you and toss up an extra meal. On a rare occasion I’ve been offered (I never ask) to use the extra bunk during the FA’s second break. There are 5 bunks on the ER and usually 9 FAs, so there may be a couple hours where 1 bunk is not being used. Be courteous and realize that the bunk is their space - we are generally not welcome down there, so nice manners and chocolates go a long way.
You’re right about the 75 minute thing. Get there extra early, especially for the return leg. Even better if you can meet the pilots/FAs in the E lounge briefing room when leaving ATL so they can all see your smiling face at once.
Leaving SCL there will probably be taxes due, which you would pay at the ticket counter when you check in. Haven’t been to scl in 5-6 years, but back then you couldn’t even get to security without being cleared by the agent.