1) In 25+ years of work experience in PR, the only limitation concerning potentially "reserved" jobs is elected positions, and that's not even a regulation or law (aside from minimum residency time), it's just a "cultural thing". If anything, being an outsider can actually be an advantage as you are immediately perceived as being more experienced/more qualified. (Not sure why this is, but it happens too often to be accidental)
2) Requirements are often listed online for everything from medical/health care professions to vocational specialties. However, finding specific information may be a pain, and phone calls to organizational or agency offices often go unanswered. Best bet: ask someone already working in the sector/profession/industry and be prepared for red tape when to comes to procedural issues
3) I have found that the best sources of information within a sector/profession/industry are those working in it, especially if they have organized a lobbying group or union, or if they are politically active
4) Most local government agencies are quite helpful when it comes to supporting new people coming to the island. All government documentation has to be provided in English, so that is a benefit. There are strong immigrant (expat) communities in the Greater San Juan area, Palmas del Mar, and Aguadilla/Rincón/Cabo Rojo, and linking with them informally can help find a source of support