HB is right take a taxi, but here's my answers to your questions.
1) Exit the airport turn left at the highway and keep going. Bypass Juan Dolio and San Pedro turn left on the highway again when you have crossed the road going from San Pedro to Hato Mayor. Unfortunately you can't bypass La Romana but have to deal with that, continue till the turnoff with the signboard to Bayahibe, can't miss that.
2) Watch out for cars, motorbikes, horses, cows, goats, guaguas on both sides attempting to overtake, overloaded trucks almost flipping over and will make a pancake of your rental car if the load hit you, potholes, three line roads that all of a sudden becomes one line roads all vehicles have to squeeze into the same line.
3) Two hours if you know the road.
4) Daylight, I usually take off six less maniacs on the road early morning.
I saw two cars off the road yesterday between San Pedro and Boca Chica plus one complete wreck on a truck on the way to Santo Domingo after a 15 min rainshower. Good luck.