Our experience is pretty much the same as already posted.
English is spoken by virtually every Greek under 60.
We too used Athens Car Rental ACR. Arrange to meet them at the airport. The drop off the car at arrivals. You do a walk around to note damage. Its a good idea to video the car. I missed a dent but my wife videoed the car and the dent was on the video so we had no problem with the inspection when we returned the car. They do take a deposit for damages greater than the deductible on the insurance they provide. If there is no damage they don't put it through on you credit card .
The airport pick up is important because you can drive all the way to Corinth on a major highway. It is a toll road so have cash with you. Generally speaking the rental cars are small and a bit under powered. They are no match for the high powered vehicles the Greeks drive. It is customary for all vehicles to stay way over in the right lane to allow fast vehicles to pass on the left. These vehicles are probably going by at about 150 km/hour.
You mention crazy drivers. I can say the Greeks love to go fast and they also cut corners. There must be a Greek gene that causes them to drive over the centre line. They all do it. It can be a bit disconcerting to go around a corner and find a vehicle straddling the center line. They always manage to get back across safely. I bring this up because after Corinth you will be on rural two lane roads not super highways.
All the road signs on the roads are in Greek but about 200 meters further along there are signs in English. Its a good idea to have a paper map and or a GPS so you have no worries about getting lost.
Tourist driving in Greece requires the passenger to be alert and actively watching for signs and following the map. The driver has to have all attention on the car and the road.
I am telling you about this not to disturb you or to dissuade you from renting the car but rather to prepare you for the experience.
I might suggest that you make a stop in Corinth on the way to or from Nafplio . Don't go into the city, the traffic is crazy. You can easily visit the canal. There is a path that runs along side of the canal. There is a ww2 pill box, an abandoned rail line across the canal and a spot where the keels of the boats were dragged across before the canal was built. You can still see the deep scratches in the rock.
There are three additional sites nearby. Acrocorinth a mountain top fortress with 360 degree views. Right next door is ancient Corinth from the biblical Roman time and lastly Ancient Nemia. It was a sister city to Olympia and still has its stadium a partially restored temple and a very good museum. It takes about an hour to visit Nemia.
Here are some images
Corinth Area. https://www.flickr.com/photos/stanbr54/albums/72157711755443483