You listed some criteria for countries suitable as offshore retreats. Unfortunately, I'm not sure there are any countries that meet all of them, in Central America--or North America for that matter! Having spent a lot of time looking into things, I'd like to share a few observations:
-Nicaragua is widely reported as having a low crime rate, plus they now offer the same immigration incentives that used to be available in Costa Rica. It is also noteworthy that most of the Nicaraguan crime is cross-border, into Costa Rica.
-Belize has a fair bit of crime, some political instability, and a bad attitude towards guns. Plus it's relatively expensive.
-Costa Rica is going down the globalist path just as fast as the US and England--they are just starting from a freer level. The recent tax laws might make it the worst in Central America. Fortunately, the Supreme Court threw them out on a technicality.
-The folks I met who were evaluating both Costa Rica and Panama leaned strongly toward Panama.
-The weather and scenery in Costa Rica are great.
-Many expats who like Mexico see it as a snowbird destination; more like a modern-day Arizona.
-The visa (e.g., Pensionado) programs are really important. Belize's retirement visa starts at age 45, which is quite generous.
Sincerely, - Pablo