Can't help with EZE, but things at SCL are pretty straightforward. You can book a taxi inside arrivals (at baggage claim) and pre-pay, and they will meet you outside customs and take you to the cab. A little more expensive than a metered cab, but worth avoiding the hassle. To get back from Bellas Artes to the airport at rush hour, just have your hotel get you a cab. Uber is another option, but it is banned at the airport so some drivers will get nervous and not want to take you. Most taxis in Santiago are metered, so scams are not really a concern. From Bellas Artes to the airport should be no more than an hour, even at rush hour, not including the construction-related traffic at the airport. I haven't been to SCL in about 9 months, so can't comment on how much you should add for that. Hope some of this helps.