Can't help you with the seat assignments as I have only flown LAN in First and Business. On those flights, I have had no problem with seat assignments well in advance.

As for the Easter Island flights, they are operated as a two-class service albeit on a three class aircraft. Unfortunately, however, business class will include both the business and first class seats, so as an economy passenger, you will not be able to sit there. This is different than when AA operates a two-class service with a three class aircraft.

It is a real privilege to be able to visit Easter Island and I am sure that you will enjoy it.