About credit cards, Us ones are accepted, because of VISA / MasterCard network but you should pay attention to the charges applied for cash advance (ask your bank in US for that).
Also ask the embassy in Athens (reachable through email) about your dual citizenship and of course as Danai mentioned before be patient with bureaucracy.
Plugs are different but as you mentioned this can be easily solved with an adapter - BUT there is also a difference in the voltage (220W vs 110W in US) which you have to pay attention in case you bring your devices from the US (all of them have switched to change to the European voltage).
Fell free in case you need something more.
Cheers for now