If you buy 2 one-way tix then you should be OK. But if you buy T+ there and Lat on the way back, then I too think the more restrictuve fare applies to the entire itinerary. Doesn't make much sense to me but there it is.

And yes - Lat is advertised as fully flexible, fully refundable but you really do need to read the T&C before buying.

One consideration would be to buy a T+ there then buy the return once you actually have some certainty, as opposed to going through changing your ticket.