Finding a gua gua from Samana to Las Galeras will not be a problem; sometimes a bit quirky, but not a problem. He can find gua guas between these places, but I'm pretty sure he won't be able to get one gua gua that will take him to them all. He will be able to get a gua gua from Samana to Limon, but he'll have to get a moto to one of the horse farms that take vistors up to the falls. Another gua gua goes from Limon to LT. And then, as already pointed out, there is time; time from arrival at one location to departure to another. I would recommend going to Samana; spend some time there and go out with Kim Beddall to see the whales; take the gua gua to Las Galeras; spend some time; return to Samana via gua gua and lease a car in Samana; go to LT; spend some time and while there do a day trip to Limon to see the falls; return the car to Samana and then back to SD via caribe tours.