In our case: Ticket issued by LATAM but starts from a European airport. All segments on same ticket -> entire trip is covered by EU261
You're right - but only if your return is operated (not sure if needs to be marketed too) by a European carrier (such as IB). If your return flight(s) is/are on LA, not all your trip will be covered by EU 261/2004, but only the part departing the EU (+ connections on the same ticket, even if outside EU). Chilean consumer protection laws would apply for instance on a denied boarding for a LATAM flight SCL-MAD - and I am not sure if this protects you much...