Arenal is about 90 kms northwest of San José; Cahuita to San José is about a 3½ hour drive. I would easily add another 2 hours to get to Arenal and this if traffic is good! A better option is to stay in Alajuela first, visit the volcano and then head to Cahuita. Going to Poás Volcano is also a great option. You can stop off at the Doka coffee plantation and the La Paz waterfalls/ animal sanctuary. Very interesting!We stayed in San José in order to do this excursion. We hired a guide and went by minivan. Good luck.
Sorry, I don't know the answer. We had rented a car because we planned to travel to various parts of the country while we were there. If you've already made arrangements to stay at Playa Negra, you could try to reach out to the owners Marise and Pierre. And give them my best!!
The Playa Negra Guesthouse has lots of birds, parrots, hummingbirds, howler monkeys and sloths. The beach is beautiful, long, and swimmable easily enough. You will be there basically by yourselves. No snorkeling though, too much surf. Besides, the park is not a long walk away either.