I drove the Amalfi coast this past February while I was based in Salerno and I wrote a little about my experiences. As you'll see, I've had a lot of experience driving coastal and mountain roads. The AC road is the most difficult I can remember - and in fact, I found driving up to Ravello from Amalfi so daunting that at some point (maybe half-way) I managed to find a spot wide enough to allow me to make what I shall euphemistically call a 3-point turn (it was a few more than that) to reverse direction and return to Amalfi. I would just add that I'd driven previously in Italy, Spain and France, am familiar with the signage, and had no trouble with a manual transmission. Much better to use the ferries and buses to get around. And, as someone else has said, Ravello is not the best place if you want to explore the other towns. You have to hike down or bus down to get a bus or ferry to go anywhere else.
Another consideration is that if they are arriving from the US, they are going to be jetlagged and sleep-deprived. Driving in that condition is the same as DUI. Add to that, the confusion of unfamiliar car, unfamiliar roads, and the like, it's a recipe for misery, if not disaster.
The best way to get to Naples, other than private transfer, is bus or ferry to Salerno and then the train, or bus to Sorrento and Circumvesuviana train. In Sorrento the bus stops at the train station, way above the seashore. In Salerno, the ferry is a short (5-10 min) level walk to the train. The bus probably stops at the train station there.
Generally the best way from Naples to Tuscany would be by fast train to Florence, then rent a car, though it does depend on where exactly they are staying.
Time allocations - remember that 3 nights = 2 days of sightseeing. They'll use a lot of their first day just getting through the airport and to the Amalfi coast. Then figure close to 1/2 day to Naples, giving them about 1.5 days in Naples itself. Then another 1/2 day to wherever in Tuscany for however many nights. Unless the flight home is afternoon or evening, I'd want to spend the last night in Rome, close to the airport. If their flight is fairly late in the day, they could drop the car in Florence and take the fast train to Rome, then the Leonardo to Fiumicino - allow 3 hours after dropping the car and getting to the train station.
Tootling around Tuscany sounds good, but they should know about the dangers of the ZTLs in many of the charming, ancient towns.
Lastly, GPS is essential for driving in Italy if you don't know the area well.
