I, like Beppington, am enamored by your story. Specifically the island itself. I found myself wide wake last night, spent till the wee hours this morn researching the area.
Oh, I would never consider making a purchase, but I could be interested in renting a vacation retreat!!
It sounds like things have really changed over the years. The island used to have daily milk delivery from Farnworth Farm (vegetables delivered every other day when in season). The casino is no longer, and it sounded like there used to be "one of most things" on the island (as far as shopping goes).
I like how each "area of houses" are referred to as colonies. There is a lighthouse on the island. Sunken ships abound, including the one just off the T wharf. A hurricane in the '40's removed a large number of houses, most never rebuilt. And SOMEONE needs to finish the gazebo at the farm!!
It looks to be a very peaceful and relaxing haven in the midst of the NE corridor. Very Nice!!
I have been working for several years to recover parts of Farnworth Farm that have been completely overgrown and to remove debris from it's days as the island motor vehicle graveyard. The farm was fully functional into the 1960s but then was allowed to go to seed. Many of the photos I've posted over the years here are of my clearing efforts there. It's about a half mile from my house and is now run by the conservancy organization that also owns all the land between my house and the bay.
The island culture was formed in many ways by the combination of isolation (the car ferry is new, the old ferry could only take one car at a time and only if the tide was right) and yet also proximity to civilization (Boston is 70 miles north). It has been viable as a place for retired couples only since the car ferry service started so before that it was pretty much only a hard scrabble group of year round residents (Swamp Yankee is our version of Redneck:laughing

and then the summer residents who as you noted tend to define themselves in "colonies" of about twenty homes or so. Until the 1990's cars did not need to be registered and made a one way trip to the island and were kept running until retired to the "parts dept" at the farm. Only one road is paved and that has been patched so many times it looks more like a quilt than road. Fixing broken things to "Prudence Island standard" is our code for baling wire and duct tape fixes that are the norm and everyone pitches in to figure out sometimes bizarre solutions for problems using only materials on hand. The Apollo 13 astronauts (Houston, we have a problem) would have had good basic training by growing up on Prudence.
The sense of isolation and community also bond folks together in interesting ways. It is considered impolite not to wave at someone, driver or pedestrian that you pass on the road or walking by. After a week on the island I have to stop myself from waving at people when I get back on the mainland and confusing them. The island rules also start the moment you step on the ferry and it is fun to watch harried people just beam the moment they board on a Friday night. The ferry also serves as a practical barrier to people coming and going so you don't spend any time on weekends doing errands or shopping (just fixing stuff). The ferry schedule also makes it very hard for someone to get on the island if they live for example in NYC. Other areas of Rhode Island are popular with NYC residents as they just drive up 95 after work and get to their vacation homes late Friday evening. If you cannot make it to the ferry dock by 7:30 you ain't gonna be on Prudence Island that night. Boston is about as far away as one could be working on a Friday and get to the ferry in time. However Bostonians are traditionally more oriented towards Cape Cod, Maine and the mountains so few come to RI. The summer weekend population on Prudence is therefore mostly from within a 70 mile radius in RI and southeastern MA and we don't have any jetsetters like Martha's Vineyard or Nantucket.
As noted earlier one of the more charming contrasts to modern American life is how intergenerational all activities are and how the island kids do not segregate themselves by age. At Halloween the community has a party for all ages and then all the kids from high school to preschool hop on the school bus (yes there is a one room school house) and go trick or treating as a group with the older kids watching out for the younger ones. Island kids are sensitive to each others potential embarrassment about playing with younger kids when they bring non island visitors for a weekend. It is not a problem if one kid with a visiting friend does not join the gang but the non island visitor kids almost always are completely engaged with the local horde by the end of the first day. The kids also get to have a 1950's experience where parents are not worried about them when they cruise the neighborhood. If they are not back for lunch the assumption is they liked the menu at someone else's house better. If you are looking for your kid you just ask any kid where the horde might be found. One might worry about the coastline except that the older kids will not tolerate the younger ones heading off alone and there is almost always some adult sitting down at the swimming area anyways.
One final tidbit. Prudence was first owned by Roger Williams, the founder of RI and a figure who is now widely recognized as crucial to the American notion that church and state are separated. Government in early Massachusetts was a fanatical theocracy. The Pilgrims were essentially Ayatollahs and religious extremists. Williams was a trained minister but had more liberal ideas about religious tolerance. He was driven out of Massachusetts within a few years of his arrival from England and went south to Rhode Island where he lived with the native population. He was helpful to the Sachum (chief) who gave him Prudence Island (used previously as a summer fishing and hunting camp by the local Indians). Williams eventually returned to England to get a charter for what is now Rhode Island from the King and specifically established the new colony to allow religious freedom. The religious tolerance of RI was in marked contrast to MA or CT so persecuted groups of colonists (Catholics, Jews, Quakers, more liberal Protestants) found refuge there and made it home. RI is the home of the oldest synagogue and Quaker meeting house in the US as well as one of the early Congregationalist and Baptist religious communities. Several of our neighbors are Roger Williams decendents.