I think you can get an up-to-date answer for 1 and 3 by calling AA. But FYI, I have a 3-continent oneworld Explorer (Asia, Europe and North America), which includes a stop in San Jose, Costa Rica.

As for 2, it depends. Some agents may be willing to offer a discount from a profit margin they get.

Just a suggestion, not an offense. Provide as much details as you can, like carriers, destinations. (Which cities in Central America you are hoping to visit? for instance.) Vague questions can get vague or no replies. Some questions can be answered by getting in touch with airlines.