The road from Santo Domingo to Samana is awesome. Great scenery and not a lot of traffic. There are tolls on it, so be sure to have plenty of money.
After you get off the toll road, there is a bit of construction taking place, but the road is not heavily travelled, so you'll be fine. My biggest frustration with driving in Samana was that the motorcycle riders stay in the middle of the lane rather than off to the right as in other places in the country.
The road from Las Americas to the toll road to Samana will probably leave your wife very nervous, but it isn't that much of a drive. My advice for that is, after you go through customs in Las Americas Airport, Brugal rum sometimes offers free samples. Get your wife to have a couple of those, and all will be well.
Also, please note, the road to Samana is off of the side road that parallels the main highway (in the US, we'd call it the access road on the side of the freeway). Be sure to be on the side road or you will not be able to get to the Samana road.