It takes 3 to 4 hours to get from Rome to the Amalfi Coast, depending upon traffic conditions.
Yes, there are a number of service areas along the route where you can stop for a coffee or to stretch your legs. There are also public restrooms.
It takes about 75 minutes to get from Naples to Positano by car, though the trip can take longer when there's heavy traffic. To get to Amalfi, it takes about 90 minutes and about an hour to get to Sorrento.