No and No
Euros
Yes. They drive on the right side of the road just like in the US and the GPS works perfectly.
The dress code is elegant, semi-formal. Meaning dresses for the ladies and suits for the gentlemen.
If you chose to present a gift at the wedding a monetary one is greatly appreciated. Transporting other type of gifts back home will be difficult.
Tough luck! Greeks don't understand the concept of being vegetarian, gluten free, dairy free or anything else. You'll be fine!