At the end of January, transactions started working like MC. It stayed in pending state for 3-4 days with the "official" rate. Before that it settled right away at the MEP rate.
No. As far as I know, it only works for Visa and Mastercard. One further tip...bring a Visa or Mastercard debit/credit that charges no fees on overseas transactions. Most will charge a further 3% on foreign exchange. Several banks in Australia now offer cards with no fees on overseas transactions. Also, Visa is quite transparent on the rate it charges on ARS: https://usa.visa.com/support/consumer/travel-support/exchange-rate-calculator.html. In my experience using an Australian-issued Visa debit card, the rate published here is exactly what you get charged on your account.