First off props to you for wanting to travel B, you should definitely do it, regardless of my recommendation, even if it's only to wack azz San Jose. In regards to your questions:
1) Limon is like the city, and Puerto Viejo and Cahuita are like the spots. Limon has a beach too, you can hang out just in Limon and have a good azz time. It really depends, if you are strictly an urban person, stay in Limon. But my G, my heart is in Puerto Viejo, that is one of the chillest spots on earth my ninja. The whole coast line is cool, but I have been to Cahuita personally, that's why I recommended it as well. Cahuita is really a small town, think of Portmore if you're Jamaican, it's very similar. Smaller, slower vibe, but what's nice is it's right by the beach, there are girls, and the food is cheap, healthy and excellent. I would spend at least one day in all 3 places, and just camp out in whichever one you like best.
2) Wherever you go somewhere, if you don't at least make an attempt to have some basic knowledge of the local language, you are missing out on life. It's tough to pickup chicks when you can't communicate, and especially if you're American, it usually ends up being some sort of transaction
. Learn Spanish B, can pick up the local chicks, save money, and have an enhanced experience. At least the basics, if the chicks see you are trying, it turns 'em on, one of the easiest way to pickup chicks is to be lookin' like you are trying to learn the language, chicks love teachin' a brother
3) My nig, I hate to say it, but if you can only travel for a weekend, stay in Miami. If you are going out of the country, and can't stay for at least two weeks, it's not even worth it. Especially in the Limon area, it rains a lot. I don't know how you can make it happen, but you gotta get at least two weeks. If you just goin' for the weekend, it will be hard to pick up a decent chick, you might as well just pay for the hoes.
4) People smoke weed in public, and the majority of the population in Limon/Cahuita/Puerto Viejo is ethnic, nobody's messing with you. But since you Jamaican, you know how the rudebwois are, a lot of the black ancestry is from Jamaicans who migrated to work there, don't disrespect people, you might get shot.
Whatever you do, if there is one thing I can't make clear enough, you need to get more time, it's a waste to travel for just a weekend.