This is insane!!
Decide if you are adventureous or not
If you are adventurous, go the Sosua, Nagua, Saman?, Sabana de la Mar, Higuey, La Romana route. It will be fun. Have someone at the hotel in Sosua write out your directions and travel lightly.
If you are more conservative,, take the Sosua, Puerto Plata, Santiago Santo Domingo route. Change buses to a bus for La Romana, maybe on Metro Tours bus-Very easy to get to from Caribe Tour in Sosua. After arriving in La Romana get a taxi to Bayahibe.
Taxi is an expensive option, but, again, you must decide if you want to stop at the different places that sell stuff on the highways! then the taxi is the very good way to go.
Not rocket science.
RE: Maps. the map they sent you to is very outdated!. As for street maps, they are not needed here since in any town the people know where things are, especially hotels and stuff like that. Of course in the larger towns there are street maps, but you won't ever need them.
You can get a good map at just about any Texaco Station.
HB